I think changing the port is not a good idea. As @ruppo wrote the proplem can reoccur with other applications. The problem is on microsoft side. They occupie with differen service dynamically the port 6556 at restart (see: MS Exchange blocked port 6556 - Troubleshooting - Checkmk Community) .
I saw this behaiviour sometimes after installaion of fixpacks or security fixes in MS Exchg environments. A stop of the given Exchg service and a restart of check_mk agenten and a start of der Exchg service fixed the problem most of the time.
Regards, Christian