Using FIleMaker 12.03 on Windows XP, I am unable to reproduce this crash. I created a brand new test file in FileMaker 11v3 with no tables, just a tab control. I tried this with two panel tab controls, one panel had a text label the second tab had no label. I was able to convert the file and then modify the tab control in tab control set up after conversion.
I also tried this with a tab label of several space characters and it also did not crash.
Let me know if you spot any differences between what I did to test this and your own expriences.
This suggests the possibility that your original file may have had some latent file damage that only came out of hiding post conversion when you tried to edit the tab control set up.
You might try running a recover in FileMaker 11 on the file and then converting the recovered copy to see if you get different results.
Today, I tryed to edit tab panel and everything works as you say. The layout was not corrupted. Yesterday, I was testing printing and there were some hangs and errors before (but it is different topic). I thought, that simple restart should be enough, but the client was not in proper state and it caused this behaviour. Restart of operating system helped.