Can not login from one client

My Clients are behind NAT, my internet connection has a fixed IP
I can login to my nextcloud from a MAC Book air, from two NOKIA smartphones but no any more from my iMAC.
I get the message “too many attempts from you address” Nextcloud is running on a Debian12 Server in a datacenter.
As I am behind NAT nextcloud should only see my official IP, like the other clients in the same private network. My browser i an current Chrome and I already cleared the cache.
Anybody any idea?

This doesn’t sound like suspicious login, but maybe brute force protection. Based on the description of your topology, your situation may justify whitelisting your NAT IP. You’ll have to decide for yourself:

If so, note the root cause really isn’t the lack of a whitelist, but whatever client is triggering things with invalid authentication attempts. Set your log level to 1 temporarily and see which IPs and clients are triggering BFP (and to confirm that’s what it actually is).

There are other possibilities as well. See the above doc link. It was recently greatly expanded.