When I opened Book Collector version 20.5.2 today, I saw the message "Failed to open database, because your license key (xxxxxxxxxxxx) is already in use on computer "YYYYYYYY" by another user (zzzzzz). Do you wish to unlock your database file and open it here?" I could choose "yes" or "no" with the "no" answer opening Book Collector with a new empty book database. The computer the message referenced was the one on which I was currently trying to open the program. Since the system I was using had crashed earlier in the day, I realized that Book Collector was likely finding a lock file that would be deleted when the program is closed normally. I saw a file with the name I had given to my book collection, but with a .bkclck filename extension in the directory where I stored the book database. When I opened the file I saw the file conained only one line:
<lockinfo><writable><client pcname="YYYYYYYY" name="5c22840efc7a3db854c44b0b780f8abc" key="xxxxxxxxxxxx" username="zzzzzzzz"/></writable></lockinfo>
In cases where Book Collector was not closed normally, you can either choose "yes" at the prompt or delete the .bkclck file before opening Book Collector as its presence is what causes the message to appear.