MacOS – Office 365 keeps prompting for login credentials on Mac OSX

First, try starting the machine in Safe Mode to re-confirm that this is a third party software issue and not related to hardware:

1. Shut down your Mac and wait 10 seconds.
2. Press the power button.
3. Immediately after you hear the startup tone, hold down the Shift key.
   You should press the Shift key as soon as possible after you hear the startup tone, but not before.
4. Release the Shift key when you see the gray Apple logo and progress indicator (spinning gear).

To leave safe mode, restart the computer normally without holding down any keys during startup.

If it works fine in Safe Mode without issues, then you could try uninstalling some potential culprits seen in the log. The backtrace in your log shows the application firewall crashing. The key suspects in this case, based on what was loaded most recently, are the following:

1. com.speedbit.driver.vadriver 1.0.8

   • Uninstall the Speedbit Video Accelerator for Mac from your system and see if it helps.

2. com.seagate.driver.PowSecLeafDriver_10_5 5.2.3 and com.seagate.driver.PowSecDriverCore 5.2.3

   • Uninstall any Seagate Diagnostics Tool that you may have and see if it helps.

Soon after writing the question, I realized that even with no plugins installed, I had the Welcome page appear, which changed what I googled for. That helped me find:

https://discussions.apple.com/thread/6663182?start=0&tstart=0

sudo find ~ $TMPDIR.. -exec chflags -h nouchg,nouappnd,noschg,nosappnd {} + -exec chown -h $UID {} + -exec chmod +rw {} + -exec chmod -h -N {} + -type d -exec chmod -h +x {} + 2>&-