After a lot of searching, including a lot of old solutions that recommended changing envvars to _envvars, I finally found the solution. Manually trying to start MAMP via console I saw this error:
httpd: Syntax error on line 83 of /Applications/MAMP/conf/apache/httpd.conf: Cannot load /Applications/MAMP/Library/modules/mod_example.so into server: dlopen(/Applications/MAMP/Library/modules/mod_example.so, 10): image not foundSo to fix the problem, I edited the httpd.conf file and commented out (#) the load module for mod_example and it worked. I've no idea why the default httpd.conf file references a missing module, but that seems to have fixed it!
Let me know if this helps you.