Cannot login to Plex server

When trying this https://support.plex.tv/hc/en-us/articles/200878643 I get an error message that “there was a problem logging in to the server”. Please advise, thanks

Device: Synology ds209
Version OS: DSM 4.2-3256
Version PMS 2.4.9

If I try entering credentials in webinterface > Server > General I get a dark popup with white letters stating There was a problem logging in to the Plex server (translated). I use Chrome and see error messages in console pertaining to Web sockets/claims it cannot establish a connection with the socket. The rest of the console messages seem fine to me: it finds plex.tv and my NAS running Plex. Hope this clarifies, thanks.

I switched my PMS over to my new NAS and could log into that without issues. Thanks for your prompt and detailed response.