>>>...at least once, I'm sure -- usually at the end of the day...<<<
once? End of the day? Then you probably lost your file forever, sorry. That's a huge risk you take every day with your workflow.

Get in the habit of manually saving often (15-20 minutes), set ISAVEPERCENT variable to 0 (that's a zero); this will give you more chances of never losing anything older than 20-minutes every again.

Read this for that error message https://knowledge.autodesk.com/support/autocad-for-mac/troubleshooting/caas/sfdcarticles/sfdcarticle...