DS918+ "this server is not powerful enough to convert video"

Server Version#: 1.19.2.2737
Player Version#:
Plex Web Version#: 4.22.3
Synology NAS#: DS918+
DSM#: DSM 6.2.3-25423 (8GB)

Upgraded from a Synology DS718+ to DS918+
DS 918+ showing error “this server is not powerful enough to convert video.” ???!!!
Supposedly they are the same NAS with just number of bays more.
I am really sad and dont know how to proceed.
IF anyone can guide to get my logs and advice what I am doing wrong.
FYI I did no change to settings just swapped and migrated to new NAS.
Also tried using a fresh install, but old files ran on the DS718+ without transcoding.
FYI some files are .avi and they no longer run on DS918+ normally, but used to run on DS718+.
Also if use any subtitle other then .srt I get the error message. Infact any use of subtitles uses higher CPU and stutters. Also some .srt dont run at all (e.g. .ass, .subs, etc.).

I tried deselectling “Disable video stream transcoding” from “Transcoder” and it seems to work but uses 100 CPU. However this was not the case before. Even with this option selected all videos including .avi and .rmvb ran normally.

But then again some run on mobile app and TV but not laptop browser. Again some only run on mobile app but not TV or web browser.

I am completely stumped and honestly sad as a great deal of money went on the upgrade.

Any suggestion would be welcome and apologies for posting in the wrong thread earlier (I assume it was wrong and praying this thread is correct)

Please guide me to get all necessary details so the community experts can suggest accordingly.

Please help