Consider this virtual breadcrumbs or a Show-n-Tell. Whatever floats your boat.
This evening, I had to upgrade a BES 4.1.7 server on Domino 6.5.2 to 5.0.2. Domino will be upgraded in a wee while, but getting all third party software up to a version that supports 8.5.2. Just so you know, if you pass 8.5.1 for your BES server, you need to go with BES 5.x for support.
Post upgrade, BES would not load and went into maintenance mode for 15 minute intervals. The error on screen was
Domino prior to R5 identified, or error occured while identifying version of Domino server. Check that the BlackBerry Enterprise Server is running on Domino R5 or greater, and that the Domino server is running properly
The problem was related to DNS naming, and server FQDN names (and DIIOP hostname field). Resolve these to the correct name, restart the OS totally and all started to work again. Additionally, check your $SavedAddresses field in your server document. You may get a shock on cached IP references for your server that are stored (we most certainly did in this case).
The single technote that gave me a hint is here.