I had this issue (and made a post) way back when FM14 was released. That issue involved ANY action that made use of a progress bar. I reverted back to FM13 because it was never resolved. I had hoped it was resolved in FM15, but apparently not. Maybe (fingers-crossed) there is a fix or work-around this time or I'm back to FM13 again.
When sorting, the progress only gets so far until Filemaker locks up. I have not tested with other actions involving a progress bar in FM15. The table in this case has a little over 3000 records and it varies as to how many it gets through before locking. The only resolution is to abort the program. Clicking cancel does not break it free.
The progress doesn't lock up the application ALL the time, but certainly MOST of the time.