There are some changes at the mendelson AS2 test server:
"key1" has been replaced by "key3", "key2" has been replaced by "key4". You should either find the new keys in your keystore (as they are delivered for some time already) or you could download them at http://as2.mendelson-e-c.com/key/key3.pfx and http://as2.mendelson-e-c.com/key/key4.pfx. The keystore password of these test keys is "test". Please do not use these keys in production! They are available to everybody.
The current keys "key1" and "key2" will expire in 08/2019.