Krasi69,
You are lucky I ran into this awhile back, but you won't like the answer probably. This was what I got from Mitel:
This affects all MiVoice Office 250 Systems with version 6.0 SP2 PR4 (6.0.11.100) or below software. Microsoft published a security update for Microsoft .NET Framework 4.5 and NET Framework 4.5.1 which changed the way Transport Layer Security (TLS) works. The Microsoft knowledge base article is KB2938782.
Mitel created a fix for this issue (MN00541309) in the MiVoice Office 250 version 6.0 SP2 PR5 (6.0.11.101) software.
MN00541309 - Fixes a PC not being able to access System Administration and Diagnostics after doing windows Security Update for Microsoft .NET Framework 4.5 and NET Framework 4.5.1 KB2938782.
To resolve this issue, upgrade the MiVoice Office 250 system to the latest available software version applicable.
-For CS Chassis the latest version is 6.1 SP1 PR3 (6.1.4.68). This is the latest version that Mitel offers for the CS Chassis which is the older discontinued chassis for the MiVoice Office 250 System.
-For the HX Chassis, the latest version at the time of this article is 6.3 SP1 (6.3.7.58).
Thanks,
TE