Messages app not loading/opening on Sonoma 14.5

***** SOLVED *****

To recap ...

I erased my SSD and reinstalled MacOS to resolve an issue with file sharing. After that, Messages would not launch.

I spent hours on the phone with Apple tech support - they were very obliging, but had no idea what was causing this problem.

I did all the usual troubleshooting steps - rebooting, safe mode, reinstalling the OS again, running Disk First Aid, resetting permissions - nothing helped.

I knew that the Message service was OK because it worked fine on my other devices - and I knew my Mac and the Messages app were OK because Messages worked fine on a newly created 'test' user account.

It had to be something in my Home folder.

Then I realised that my freshly re-installed MacOS was missing Microsoft's Calibri font. I reinstalled that, then Messages instantly started working again !!

This explains everything !!

I'd previously set the Text Size in Messages/Settings/General to Calibri 14pt, and this was embedded in the Messages preferences in my Home folder - which explains why Messages (which defaults to the System font) worked for the 'test' user, but not for my User account.

Apparently, Messages looks for the specified font - if it can't be found, then it just gives up.

Unfortunately, this makes it impossible to trouble-shoot - or to fix easily - because if Messages won't launch, you can't edit the font settings.

So - that explains the cause.

The solution now is for Apple to explain how to reset the font settings for the Messages app when the app can't be launched.

I hope that helps :)