the error in Plex Media Server.log was:
error: unable to set up server: failed to load certificate file ‘c:\program files (x86)\plex\plex media server\resources\cacert.pem’
the solution was to make copy of the dh512.pem and rename it to cacert.pem like mentioned here:
Renaming of the dh512.pem is probably not the best way to solve it.
In the end I have downloaded the latest version installer PlexMediaServer-1.18.3.2129-41af4e729-x86.exe and upgraded. After that I could see that the cacert.pem is 205KB in contrary to the dh512.pem which is just 1KB.