Good morning,

 I have a problem. I have an HMI Unified MTP400 panel with firmware version 18.0.0.3 and an S7-1200 1212C DC/DC/DC PLC with firmware V4.6. I tested the program, and everything was working fine. Then, it was sent abroad, and after starting it up, there is no communication between the PLC and the HMI. An error appears: "Connections: Cannot connect to PLC - Open aborted due to invalid state of S7PlusConnections." I tried connecting remotely, setting the time on the PLC and then on the HMI, but it didn't make any difference. Do you have any ideas?

I had this issue recently and got help from Siemens support to solve it.

What we did: Upgraded to Tia V18 Update 5, wincc unified runtime update 5 (was running RT client).
Deleted the certificates under  “Protection & Security > Certificate Manager" 

Then ran the “Security wizard” under “Protection & Security” tab.

Did all the steps in the “Security wizard”, wizard added a new certificate, and the alarm was gone. 
I tried many times to add the certificate in the “Certificate manager” manually but that didn't work. Only when i ran the “Security wizard” it worked.
 

Br