Invalid base64 string for MIME content

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

First, ensure you have a complete and current backup. Then… Please download and run EtreCheck and share the report to the clipboard. Then open a new reply here, and then press the button that looks like a printed page to get a text input (attachment) box to paste the hardware and software configuration report here. From that, we might be able to spot a hardware or software issue with your Mac.