Okay, so i have found another bit of time to do some more troubleshooting:
Logs and TcpDumps at the End of the post.
Same as above:
I have deleted all Logs from PMS and added an new TV Shows Library called anotherTEST:
Media Server.log
May 08, 2015 11:38:44:159 [5016] DEBUG - Request: [127.0.0.1:5969] POST /library/sections?type=show&agent=com.plexapp.agents.thetvdb&scanner=Plex+Series+Scanner&language=de&name=anotherTEST&location=K%3A%5CanotherTEST (3 live) GZIP
May 08, 2015 11:38:44:159 [5016] VERBOSE - * type => show
May 08, 2015 11:38:44:159 [5016] VERBOSE - * agent => com.plexapp.agents.thetvdb
May 08, 2015 11:38:44:159 [5016] VERBOSE - * scanner => Plex Series Scanner
May 08, 2015 11:38:44:159 [5016] VERBOSE - * language => de
May 08, 2015 11:38:44:159 [5016] VERBOSE - * name => anotherTEST
May 08, 2015 11:38:44:159 [5016] VERBOSE - * location => K:\anotherTEST
May 08, 2015 11:38:44:159 [5016] DEBUG - Auth: We found auth token (xxxxxxxxxxxxxxxxxxxx), enabling token-based authentication.
May 08, 2015 11:38:44:159 [5016] DEBUG - Auth: Came in with a super-token, authorization succeeded.
May 08, 2015 11:38:44:160 [5016] DEBUG - Flushing directory modification time cache.
May 08, 2015 11:38:44:172 [5016] DEBUG - It took 0.0 sec to serialize a list with 1 elements.
May 08, 2015 11:38:44:173 [5016] DEBUG - Library Updater: Requested that section 52 be updated, force=0, deep=0, subdir= iTunes=0
May 08, 2015 11:38:44:173 [5016] DEBUG - Library Updater: Queueing section 52
May 08, 2015 11:38:44:173 [5016] INFO - LibraryUpdateManager path watching is disabled
May 08, 2015 11:38:44:173 [1760] DEBUG - Completed: [127.0.0.1:5969] POST /library/sections?type=show&agent=com.plexapp.agents.thetvdb&scanner=Plex+Series+Scanner&language=de&name=anotherTEST&location=K%3A%5CanotherTEST (3 live) GZIP 0ms 715 bytes 201
May 08, 2015 11:38:44:174 [10764] INFO - Notification: Scanning the "anotherTEST" section
And yeah if it helps in this Case, my library language is always German.
com.plexapp.system.log
2015-05-08 11:38:44,657 (25c) : DEBUG (runtime:717) - Handling request GET /system/agents/search?mediaType=2&id=188513&identifier=com.plexapp.agents.thetvdb&duration=-1&episodic=1&filename=K%253A%255CanotherTEST%255COutlander%252ES01E13%252EThe%252EWatch%252EGERMAN%252EDUBBED%252EWS%252EWEBRip%252EXviD%255COutlander%252ES01E13%252EThe%252EWatch%252EGERMAN%252EDUBBED%252EWS%252EWEBRip%252EXviD%252Eavi&lang=de&openSubtitlesHash=2be85d27541bb4e4&plexHash=d29d2e5ed469615406f11e7c88de05d34f6ee996&episode=13&episodic=1&name=&season=01&show=Outlander%2E&year=
2015-05-08 11:39:17,040 (1218) : DEBUG (logkit:13) - Writing data for metadata://seasons/1/posters/com.plexapp.agents.fanarttv_9d2fd05ab4561ec688c7598f722048ef3b618875 (in com.plexapp.agents.thetvdb)
2015-05-08 11:39:17,042 (1218) : DEBUG (logkit:13) - Recombining the metadata bundle for com.plexapp.agents.thetvdb://270408/1?lang=de (TV_Show)
2015-05-08 11:39:17,046 (1218) : DEBUG (runtime:918) - Response: [200] str, 487595 bytes
com.plexapp.agents.thetvdb.log
Those are the Images from the TVdb
I do see them on my PMS:
However, the used Seasonposter is the one from Fanart.tv which is the following.
com.plexapp.agents.fanarttv.log
So the used Seasonposter is the following:
Which looks like:
This one is already used within the Season as Poster, however the Thumbnail is missing again.
The TCPDump also shows that we get from Fanart the correct Link per JavaScript Object Notation (json):

Also the HTTP Get is correct, we do see the 3 way handshake (syn - syn Ack - Ack) and the http get to the correct link, also the webserver from fanart responds with an OK and the jpg:

Session closing is also fine with an FIN ACK.
Further thing i have checked, i DO have the Picture on my HDD, as stated in the LOG:
Plex Media Server.log
May 08, 2015 11:39:35:370 [3916] DEBUG - Calling back into ourselves for photo to transcode, optimizing the process (status: 302)
May 08, 2015 11:39:35:596 [3916] DEBUG - Saving original media file to F:\Plex Media Server\Cache\PhotoTranscoder\78\78ad632d95db719cb9bc8c6d5987950a675393ed.jpg
May 08, 2015 11:39:35:623 [3916] DEBUG - Created thumbnail of size 158x225, has pixels: 1
May 08, 2015 11:39:35:629 [1760] DEBUG - Completed: [127.0.0.1:6308] GET /photo/:/transcode?url=http%3A%2F%2F127.0.0.1%3A32400%2Flibrary%2Fmetadata%2F188514%2Fthumb%2F1431077975%3FX-Plex-Token%3Dxxxxxxxxxxxxxxxxxxxx&width=150&height=225&minSize=1 (7 live) GZIP 15ms 9430 bytes 200 (pipelined: 4)
May 08, 2015 11:39:50:043 [1760] WARN - We didn't receive any data from 127.0.0.1:6306 in time, dropping connection.
May 08, 2015 11:39:52:605 [1760] WARN - We didn't receive any data from 127.0.0.1:6301 in time, dropping connection.
May 08, 2015 11:39:52:711 [1760] WARN - We didn't receive any data from 127.0.0.1:6304 in time, dropping connection.
May 08, 2015 11:39:52:742 [1760] WARN - We didn't receive any data from 127.0.0.1:6305 in time, dropping connection.
May 08, 2015 11:39:55:630 [1760] WARN - We didn't receive any data from 127.0.0.1:6308 in time, dropping connection.
It seems that the Created thumbnail of size 158x225, has pixels: 1 does not either work correct, or it is not correctly saved or something..
It seems to be defenetly within this Thumbnail creaton. I think i have excluded everything else from my side.
Not sure how this is done, but maybe there should be a bit more delay between the connection of the dynamic high port and the transcoding?
EDIT: I need to correct me, it seems that the file IS there on the HDD:
i took the following line from the com.plexapp.system.log
2015-05-08 11:39:16,665 (1218) : DEBUG (logkit:13) - Downloading data for metadata://seasons/1/posters/com.plexapp.agents.fanarttv_9d2fd05ab4561ec688c7598f722048ef3b618875 (in com.plexapp.agents.thetvdb) from
I used the text marked in orange and looked it up.
The result is the following, i got that the content is saved here:
F:\Plex Media Server\Metadata\TV Shows\0\1a9c584977da385e7fc532bf03adb7c4a585567.bundle\Contents
I have lloked in the Season xml file and have seen the following:
<item external="True" url="<a data-ipb='nomediaparse' href='http://assets.fanart.tv/fanart/tv/270408/seasonposter/outlander-54dfc23385c1f.jpg'>http://assets.fanart.tv/fanart/tv/270408/seasonposter/outlander-54dfc23385c1f.jpg"preview="com.plexapp.agents.fanarttv_9d2fd05ab4561ec688c7598f722048ef3b618875"/>
<item external="True" url="<a data-ipb='nomediaparse' href='http://thetvdb.plexapp.com/banners/seasons/270408-1-2.jpg'>http://thetvdb.plexapp.com/banners/seasons/270408-1-2.jpg"media="com.plexapp.agents.thetvdb_dea84d1874bb1dd6495aefd93aed19b7efaa3684" sort_order="32"/>
<item external="True" url="<a data-ipb='nomediaparse' href='http://thetvdb.plexapp.com/banners/seasons/270408-1.jpg'>http://thetvdb.plexapp.com/banners/seasons/270408-1.jpg"media="com.plexapp.agents.thetvdb_f3de864d4da9135ff5c28c630c71ad9c54df8716" sort_order="33"/>
<item external="True" url="<a data-ipb='nomediaparse' href='http://image.tmdb.org/t/p/original/yE9EvhZh8mOPmPh4ZqApea4JHpn.jpg'>http://image.tmdb.org/t/p/original/yE9EvhZh8mOPmPh4ZqApea4JHpn.jpg"preview="com.plexapp.agents.themoviedb_b4d35fbfdc6bc5aa7b39c2b155479cd4d4cdb8a6"/>
The Locations i have found are:
F:\Plex Media Server\Metadata\TV Shows\0\1a9c584977da385e7fc532bf03adb7c4a585567.bundle\Contents\_stored\seasons\1\posters\com.plexapp.agents.fanarttv_9d2fd05ab4561ec688c7598f722048ef3b618875
F:\Plex Media Server\Metadata\TV Shows\0\1a9c584977da385e7fc532bf03adb7c4a585567.bundle\Contents\com.plexapp.agents.fanarttv\seasons\1\posters\9d2fd05ab4561ec688c7598f722048ef3b618875
F:\Plex Media Server\Metadata\TV Shows\0\1a9c584977da385e7fc532bf03adb7c4a585567.bundle\Contents\_combined\seasons\1\posters\com.plexapp.agents.fanarttv_9d2fd05ab4561ec688c7598f722048ef3b618875
^This one conatins all other Seasonposter Art which is displayed correct.
F:\Plex Media Server\Metadata\TV Shows\0\1a9c584977da385e7fc532bf03adb7c4a585567.bundle\Contents\com.plexapp.agents.fanarttv\seasons\1\posters\9d2fd05ab4561ec688c7598f722048ef3b618875
This seems to be the Thumbnail File for the Poster:

The dimension of this is 200*285 which seems to be the Thumbnail.
The Issue seems to be related to the Web Server (JavaScript )directly:
This is the Series Poster, we can see it uses the FanArt Poster correctly:

This is the Season Poster, we can see it uses the FanArt Poster but does use an Placeholder rather than the image:

You can find the Logs and TCPDumps here:
https://drive.google.com/folderview?id=0B-V8E6vybl0CfjdnSTA0Z1ZwejhhR2t2MnlFNkpjem5yM3EwRkxMWGNXVnpJRy1BbmpLNTA&usp=sharing
Hope this helps to find the Issue :)
cheers zuli12