Eek! I just tested MaxIm 6.21 and the FITS headers I am getting a good Date-Obs, e.g.:

DATE-OBS = '2020-01-23T18:38:10.95' /YYYY-MM-DDThh:mm:ss observation start, UT

So this is weird! Immediately after downloading the image, it is opened in PinPoint and solved. The error message you see is from PinPoint, it can't understand the DATE-OBS info and has discarded it. This is strange, even stranger is the info that is in there, seems like some random trash from memory. My test was with the MaxIm camera control. So as I think about this, I ran a test with ACP and the camera simulator.... and now I can see the additional FITS "synonyms" that PinPoint always writes into the header:

DATE-OBS = '2020-01-23T18:50:29.080' / [ISO 8601] UTC date/time of exposure start
DATE = '23/01/20' / [old format] UTC date of exposure start
TIME-OBS = '18:50:29' / [old format] UTC time of exposure start
UT = '18:50:29' / [old format] UTC time of exposure start

I also note that the MaxIm FITS dump you posted (image) is from the temp file created by ACP, presumably because ACP was stopped in its tracks by not being able complete its process.

In any case, from what you posted it seems like this is really coming from MaxIm (two separate data points, where it appears and replacing with 6.20 clears it). Hopefully the data above will provide at least some useful info.