I'm having the same problem. Trying to sign in from the preferences of media sever v 0.9.7.28 (OS X 10.8.4) and I get "there was a problem signing in to myPlex". I can successfully log in using the same u/p from my clients, eg Plex/Web 1.2.2, and on https://my.plexapp.com/.
The problem is new - I was actually already logged in from the media server. While trying to document the process for my brother who is also having troubles logging in, I logged out, and now I can't log in again.
The Media server log shows (my u/p removed - my username is not an email address, in case that is relevant) :
Jul 14, 2013 11:33:56 [0xb1050000] DEBUG - Request: PUT /myplex/account?username=***&password=*** [127.0.0.1:57279] (3 live)
Jul 14, 2013 11:33:56 [0xb1050000] DEBUG - * username => ***
Jul 14, 2013 11:33:56 [0xb1050000] DEBUG - * password => ***
Jul 14, 2013 11:33:56 [0xb1050000] DEBUG - HTTP requesting to: https://my.plexapp.com/users/sign_in.xml
Jul 14, 2013 11:33:57 [0xb1050000] DEBUG - MyPlex: Did update authToken for '***' (returnCode: 401)
Jul 14, 2013 11:33:57 [0xb1050000] ERROR - MyPlex: Got error: Invalid email, username, or password.
<?xml version="1.0" encoding="UTF-8"?>
<error>Invalid email, username, or password.</error>
Plex/Web does not seem to keep logs, at least there is nothing in ~/Library/Logs/ and nothing mentioned in the FAQ. Nothing shows up under "Activity" (...#!/notifications).
Yes, I am sure my u/p are correct. I also use Plex/iOS and Plex/Android and they are both logged in. I am not game to try logging them out in case I cannot log in again.
Is this a server-side issue ? Any help appreciated.