This happened to me. I never did figure out the cause. One day customers of www.websequencediagrams.com started emailing me saying they couldn't access their files. Turns out it was corrupted and would just error when accessing certain records. Also, for mysterious reasons, there was a single open transaction that had been accepting all the data for several days, so I had to be very careful when restarting the app...

Coincidentally, the backups had stopped working a couple of months ago.

Fortunately I was able to copy the data to my machine, write some python to try to retrieve each customer's data individually, verify its consistency and merge with the older backup so most people didn't notice.

Afterwards I upgraded to the latest sqlite, as the one I had been using was six years old, and I have not had a problem since.