What strange was, I did a dalvik and cache wipe before installing the update 4.20 and the play store was still there. I never had that with an update. But now with every restart the old market keeps returning. I will try it later with a wipe of the market cache. See what happens then.
Just an update: the problem is not solved with 2.5.4 on SGS2. I thought i read somewhere on this forum that there's an updated version of the market in the rom.
That's normal behaviour. Everytime you open your market for the first time, it begins to download the latest version and then automatically updates itself in the background.
A solution (got it from xda I think) that worked for me was to first delete phonesky.apk from /system/app. Then reinstall playstore and move the apk from /data/app to /system/app. I have rebooted several times after and have not seen it revert to an old version