Remote Partner closes Connection (EERP issue ?)

Hello all,
I'm using the free OFTP2 client (thx to mendelson for offering that) since 2017 for exchanging (just receiving) data with 2 clients).
Recently (June 2023) likely (not disclosed) one of the customers has updated their OFTP2 client, since then fetching data from them or receiving fails in the postprocessing (waiting for EERP/NERP) - meanwhile I have updated to the most recent version (1.47), checked all certificates (and made a new one for our side just in case there is a certificate generation issue) - checked all root certificates and certificate pathes - no success so far.

Polling for data causes the remote host to close the connection (and then yielding an error from JAVA) - EDI expert at the customer side claims that their error message indicate certificate issues.

The customer claims that the issue must be on the mendelson side as they can exchange data with others - and exchange with them (same certificates) with another OFTP2 client (eval version) works fine - so its potentially a setting or TLS (issue (TLS 1.3 ?)).

Even more confusing - with the other customer, all works fine, even with encrypted and compressed data.

Anyone an idea where to look into ?

thx and kind regards - Ralf