MS Access type databases, like the one used by default by ThumbsPlus, have a 2GB size limit. You must be using pretty large thumbnails to cross that limit with only 38K thumbnails, though, or maybe you don't use JPG compression for the thumbnails? Personally I use 160x160 pixel thumbnails, with JPG compression at 'Q88' (quality level 88).
If you can't compromise on the size or compression of the thumbnails, you could use the SQLite3 database format instead. This is supported by ThumbsPlus too (See Database | New) and it has no size limit, other than any size limits imposed by your disk format and operating system. Out of the box, SQLite3 is slower than the regular MS Access database, but you can speed it up dramatically by using an alternative SQLite3 driver, as discussed [msg=4942]here[/msg].