Hi desginbengel,
I encountered the same issue after the update.
Possible reason for this issue:
The content Type "Message" has been removed from seblod core in one of the newer versions.
Unfortunately the updater does not deactivate the old storage plugin of the message content type.
AS the old plugin seems not be be compatible with Seblod 3, you get the error message. (My backend was not working anymore).
Solution approach (which worked for me):
I went to the Joomla "Extension Manager > Manage"
And then I searched for the seblod extensions (cck) with version 2.3. One of them is CCK Storage Location Joomla Message. If you deactivate the plugin, the issue should be gone.
One additional remark: I recognized that some other Seblod 2.3 extensions were still active (Div Clear and something else). To make sure, that everything works, I deactivated these extensions as well.
Hope this helps!
Best Regards
sUrfa