So how can I update Plex when I want to stay with OMV 2 for a longer time? I presently don't want to upgrade to OMV 3 because I don't have so much time for it.
The currently available debian wheezy OMV 2.x compatible Plex Media Server package is v1.75.4035.
@yakumo, the individual who has been providing these packages ceased providing any PlexPass packages, so there are none available more recent than v1.75.4035.
If you have a PlexPass, then you have access to more recent Ubuntu Linux packages and you can convert these yourself for use on OMV 2.x using instructions @yakumo provided somewhere in his Plex.tv forum thread:
Make sure you have a copy of your currently working Plex package so that you can downgrade back in case your converted package does not work.
I used to convert the Ubuntu packages myself for my own use when @yakumo got behind schedule in providing them to us, but since I do not have a PlexPass myself, I can not offer any more recent packages.
So, in summary, either wait for for Plex to release an updated publicly accessible Plex package and hope that @yakumo or someone else converts it for use on OMV 2.x, or convert it yourself.