There may be a DB Configuration issue. You could try:
1) Opening SIM and clicking the repair button next to Inventory Solution
2) Manually reconfiguring the Inventory Solution using AexConfig.exe /configure Path-to-inventory.config file
It has been awhile since I have worked with Inventory and I don't recall the exact name of the inventory.config file. Maybe someone with Inventory Solution 7 could comment?
The other option would be to try an Aexconfig /configureall, but that can be a bit risky (It sets some items back to their default settings and could cause problems with Patch Management solution if you are running Patch Management SP1 (SP2 is not affected by this issue).
Please let me know if you have any questions.