Oh no! (bangs head on desk). We had moved the data location to D:\Symantec\.... a few years ago!
Thanks for pointing me to the sesmlu.log, which for future reference is located in C:\Program Files (x86)\Symantec\Symantec Endpoint Protection Manager\tomcat\logs.
This log contained this path with D:\, and gave away the correct inbox dir.
Thanks a lot for the help! It immediately created a directory when I dropped in the jdb, but the defs aren't updated yet. Anyway, if it doesn't update, that will be a different problem.