several problems after migration from g1 and upgrades of g2 core

Hi all,

first of all, I want to thank everyone participating in the development of this superior software!

now, my problems are the following:
After several upgrades (during the last few years), beginning with an imports from gallery1, I am finally facing several weird problems with gallery2 embedded into a cms (zikula). I am currently involved in the development of this integration bridge, and have tested it successfully with several fresh installtions (while its still far from complete). However, with my own installation as described above, I am getting errors I cant explain/understand. They really only occur in my specific case, in embedded mode with a gallery that has been upgraded several times.
For example, when accessing gallery2 from within the cms, login out, and loging back as another user. When I then access the module again, everything looks ok, except the images are not displayed at all. They HTML looks correct, they are just not displayed. I figured out a 'workaround' by accessing http://domain.tld/index.php?module=mGallery2&g2_highlightId=123. This seems to successfully initialize my session.
Another symptom is, when accessing the Plugins page from the admin panel, it breaks out from the embedded view somehow (not redirecting), and just displays the sidebar.
Yet another example is, I am just writing a module to let a user change his password for different applications of the website (at once). Therefore I am also making use of GalleryEmbed in the common sense:
first initialize, then updateUser, then finish. This works as expected in a fresh installation of gallery2, but not my original. Here I am getting the following error:

Error (ERROR_OBSOLETE_DATA) : UPDATE g2_Entity SET g_modificationTimestamp=?,g_serialNumber=? WHERE g_id=? AND g_serialNumber=? (1247278089|10|26|9)

* in modules/core/classes/GalleryStorage/GalleryStorageExtras.class at line 362 (GalleryCoreApi::error)
* in modules/core/classes/GalleryStorage.class at line 324 (GalleryStorageExtras::saveEntity)
* in modules/core/classes/GalleryEntity.class at line 294 (GalleryStorage::saveEntity)
* in modules/core/classes/GalleryUser.class at line 218 (GalleryEntity::save)
* in modules/core/classes/GalleryEmbed.class at line 448 (GalleryUser::save)
[snip]

I did integretiy checks, and all kinds of maintenance things.
Now, I am really lost. So, instead of studying the g2 core, I thought to ask here first. Yes, I searched the forums (also on google), but with no luck.
So I would ask the following:
Does anyone know whats going on?
How can I best start from scratch, but keep data like comments, descriptions etc?

Help

Thanks in advance,
Philipp