Hi @magicmiket,
Can you please try disable one by one below feature and cross check if any have conflict with login functionality.
1) Deny bad query string – WP Security > Firewall > PHP rules tab. Deny bad query strings: uncheck the checkbox and save
2) Advanced char filter – WP Security > Firewall > PHP rules tab. Enable advanced character string filter :uncheck the checkbox and save
3) 5g firewall rules – WP Security > Firewall > 6G Blacklist firewall rules tab. Enable legacy 5G firewall protection uncheck the checkbox and save
4) 6g firewall rules – WP Security > Firewall > 6G Blacklist firewall rules tab. Enable 6G firewall rules – uncheck the checkbox and save
5) REST API – WP Security > Firewall > WP REST API Can you please cross check the Disallow unauthorized REST requests:
If disabling above do not solve issue. Please cross check below option also.
Do you have manual approval for user registration on?
WP Security > User security > Manual approval tab or
WP Security > User security > Salt tab – salt postfix enabled please disable it and check
WP Security > User security >Force logout tab – disable force logout and check
Regards