What version of VistaDB are they using?

Have they monitored the size of the database file to see if it is growing? (And also monitor the space available on that drive to see if it is being used up.)

If it is back to working after "exiting" it could possibly be a temp file growing insanely and then getting automatically deleted when the connection is closed.  They might check the user's Temp folder (C:\Users\username\AppData\Local\Temp)--sorted by most-recently modified to the top, or possibly by size--and see if a temp file with a name like "tmp????.tmp" (where ? is a hexadecimal digit) is growing while using VistaDB (and if it specifically gets deleted when the VistaDB connection is closed).

There are issues with certain models of SSD drives not following expectations for file I/O such as "lying" about having flushed writes when they haven't or possibly getting reads and writes out of sequence.  But I don't know how such issues could specifically cause this symptom.  Of course, if the SSD drive has a smaller capacity than what had been used before then it could be that it previously had enough space to ride it out but now runs out.