I suggest uninstalling MainType, removing the databases when asked if you wish to, then reinstall and rebuild your font libraries.
If you have tens of thousands of fonts or don't want to do that for some reason, then wait for a reply from Erwin to see if there's a solution that doesn't require rebuilding the font database.
In my case, with only about 3,000 fonts it is not a time-consuming task to reinstall and rebuild the database.
From the error message, it looks to me as if some disk error occurred (old or failing hard disk, or a power interruption perhaps?)
I'd prefer not to rebuild but if that's the only answer.
The drive it's on is less than a year old so I hope its not dying and I have a UPS so I doubt that's the issue. I ran quite an aggressive virus scan after the last time I closed MainType...I hope it's not related.
I'm sorry to hear you've been having this issue with our software. A complete reinstall will indeed solve the problem, but you will loose all your custom groups, and ratings if you assigned those.
We might be able to solve the problem, but it requires access to your computer. I'll contact you to discuss this further.