Error Message 26 : 400

Thanks Heaps! Figured it out when I disabled the Cinema4D software. 

Sounds liek an out-of-bounds memory error. You may have to check security settings on your Mac. For anything beyond that you will have to provide info on actual render settings, contents of the composition, what Mac you're actuaslly on, version of OSX and so on. Again, I think this is some security mechanism of the system blocking a specific function, not directly an AE error, but anything is possible.

Mylenium

It looks like a permission error. Would you run After Effects by Run as Administrator.

C:\Program Files\Adobe\Adobe After Effects 2021\Support Files\AfterFX.exe <- Right-click by mouse.