Sorry for necroing the thread - just a hint for future users experiencing this issue: in my case it was a leftover /volume* folder. Volume1 had existed some time ago in my Synology configuration, but was deleted a while back. However for some reason the folder /volume1 was still there and occupying quite a bit of space (~500MB) with @syno and other folders / app cache, etc. I deleted the folder, restarted and everything was good. Only package that went missing is the synogear / Diagnostic tool package which appeared to have been installed to volume1.
TLDR: check for remnants of old volumes (like /volume1 etc) which are no longer in use, delete the appropriate folder, check your packages afterwards if everything is still there. Ah yes, be sure what you are doing - if you delete the wrong /volume folder you will delete your data 😁