The first thing I tried was reindexing, i.e. File Maintenance > Reindex, then > Optimize, then > Validate File Integrity. However, this workaround did not fix the problem for me. (In fact, the "Open" window to the TMG .dbf files appears after the Validate is complete.)
I also tried deleting my index files, as you did, and still had the problem.
This is happening on two of my four projects when I updated to 6.04. The two projects with problems are medium size with about 3,000 names each. The two projects without problems are small, under 200 names each.
Thanks for your feedback.
Earl