When rebuilding thumbnails from maintenance, digikam UI is blocked and unresponsive. No sign of activity (or "busy" indication) in the UI. It should be handled as a normal background job, or at least show a modal progress dialog.
typically, this problem must be fixed with 5.3.0. Please test and feedback with digiKam beta 5.3.0 AppImage bundle. https://drive.google.com/open?id=0BzeiVr-byqt5SUZRMGtWa3VCUDg Gilles Caulier
With the AppImge the problem seems to be gone. There is still an initial bunch of seconds of unresponsiveness, but my about 70K pics are on an USB3 drive so it may be I/O related. After those seconds though the job is started correctly.