Reporting back in: got it all fixed thanks to all the suggestions here.
Situation: my Exchange server uses a certificate for a different FQDN, than it is required for me to use for access from the Internet.
My symptoms were: Exchange Calendar and Contacts internal and external server settings kept on "self-modifying" from the name I configured in the configuration dialogs to an inaccessible (and via DNS also unresolvable) hostname, that the Exchange server's X.509 certificate was issued to (CN). This self-modifying behaviour appeared like this: 1. I change the settings of the internal / external servers to the working ones, can verify the access, 2. quit the application, 3. restart the application, experience "self-modified" server settings to non-functional results, probably "auto-configured" based on the Exchange server's X.509 certificate properties. As a result Mail access was always possible and only the Calendar and Contacts access was repeatedly getting lost, which was a huge annoyance.
Following measures were taken:
* added Domain in front of username in Mail account settings of the Exchange account
* deleted the Exchange entries with domain-less usernames from the keychain
* inactivated the "Auto Discover Service" in Mail account settings of the Exchange account
* corrected all settings once again, quitted the affected applications
* also restarted the Mac
Result: configurations now keep being intact. Both seems to have worked on two almost identically configured MacBooks (Air and Pro 15, late 2011), latest and greatest software installed.
I didn't verify which of these measures that I've taken and listed above were really the ones responsible and necessary to achieve the desired results - all I can say, it works now, and thank you!