RBL Check errors when verifying a domain with nameservers on Cloudfliare

[redexperts2023]

Every day we receive a message from RBL's checks informing us that no responses were received from some spam checklists.

The content of said message shows us several errors similar to this:

Error: open resolver; https://www.xxxxx/returnc/pub/xxxx.xx.xx.xx

The IP address at the end of the URL is an IP address that belongs to Cloudfliare.

This only happens with 2 of the 3 servers that I manage, in these 3 servers the RBL verification is configured.