@JKSH I tried a few things and it seems it's an access violation error. Normally it should crash but I think it is an unhandled try-catch block. Probably I am going to do the same on this machine too that is, Delete Qt folder manually and install 5.5, and hope that this issue is fixed in the new version :)
How did you edit MaintenanceTool.dat file? It's a binary file containing xml data and some binary characters between tags. When I remove a single character in it running MaintenanceTool shows errors like this:
Cannot seek to -15118 to read the operation data.
I don't remember exactly. I changed only XML parts, and it worked for me. But that was before the major update to the Maintenance tool that came recently. Maybe they added a CRC check on the file, or something like this.