I have had such a bug for some time, i had changed my dedicated server but i dont know if that error was not earler, can not verify this now. is it possible that other plugins could block it? ImageLibrary? (it had some time ago update)

(10:04:02) | [Steam Groups] Polling Steam group(s) to get member list
(10:04:02) | [Steam Groups] Checking Steam group xyz, starting from page 1
(10:04:02) | [Steam Groups] Steam is currently not allowing connections from your server! code=429. Aborting this call
(10:04:02) | [Steam Groups] Backoff polling state enabled

This means your server hit a rate limit, so it'd need to wait.