Hi,
Whenever you get locked out as the result of Shield, you can use a forceoff method outlined in this guide here:
https://support.getshieldsecurity.com/support/solutions/articles/3000000959
This will keep plugin active allowing you to get back in and change settings (i.e. remove your new created login url).
Regarding the problem you described, are you using page cache plugin/system?
Often caching can cause problems if it’s misconfigured or the cache is stale.
Our suggestion if you’re going to use Caching – if anything ever stops working disable and clear your caches and then check functionality. If it’s working again, try re-enabling your cache.
You can read a bit more about this approach here:
https://onedollarplugin.com/blog/5-golden-rules-wordpress-site-optimisation/
As you also mentioned an issue with brute force attempts, maybe this can help:
https://getshieldsecurity.com/blog/shield-wordpress-brute-force-ddos/
Also, you may want to read about our Antibot Detection Engine here:
https://getshieldsecurity.com/blog/wordpress-anti-bot-detection-engine/
Hope this was helpful for you in some way.
Thanks and have a nice Sunday.
Jelena
