*Server refused public-key signature despite accepting key!*
https://www.openssh.com/txt/release-8.8 says
Potentially-incompatible changes ================================ This release disables RSA signatures using the SHA-1 hash algorithm by default. This change has been made as the SHA-1 hash algorithm is cryptographically broken, and it is possible to create chosen-prefix hash collisions for <USD$50K [1] ... Incompatibility is more likely when connecting to older SSH implementations that have not been upgraded or have not closely tracked improvements in the SSH protocol.
Can you update your SSH client to a newer Version?