I know the best practice is to get all your servers on the same CU level and to update the mailbox servers first. However, I'm in a situation that makes me a little nervous and don't have a easy way to test.
I have an Exchange 2013 DAG with 2 nodes running the mailbox role, and have 2 seperate CA role servers. The environment does coexist with Exchange 2007 and we ran into the problem KB2997209 and had to install the interim update.
Currently, the 2 CA servers and 1 of the mailbox servers is still running CU5. I activated the DBs on the mailbox server with CU6 interim update and users that use OWA were able to login but none of the button fuctionality worked. (e.g. if you click on Reply or New Mail nothing would happen) Another user reported that they couldn't open attachments. So the DBs were activated back to the mailbox server with CU5 and it works.
So, my issue is I don't want to update the other mailbox server because OWA would not work. But I have a feeling that updating the CA server(s) before the other mailbox server will resolve the issue.
Has anyone seen this issue or have any advice?
Thanks!