I have a database with a MainGrid and 3 "SubGrids (with Tag Sources):

1. To delete SubGrid3, I deleted it from the Manage Grids dialogue. 
2. SubGrid3 is then still there, but all Gray.
3. Close the database and reopen => I get a message that SubGrid 3 cannot be opened. Click OK.
4. Switch to SubGrid2 and then back to SubGrid3 => I get the same error. Click OK. => IQ "crashes" (Exits).

My error was that I didn't remove/close the SubGrid3 from the MainGrid before deleting it. If I do this, I will not have this issue.
I could also close SubGrid3 before step 4 above to avoid that IQ Crashes/Exits.

p.s. Pierre, I have E-mailed you two databases : Delete_SubGrid_TEST.SNDB (before deleting the SubGrid) and Delete_SubGrid_TEST1.SNDB (after deleting the SubGrid)