Server cannot connect to Client

This is kind of normal behaviour that leads to some issue with your license.
Are you using hardware key or software key?

Please consider contacting support.

Are you running Citect2018R2?
Have you recently applied an update?

We have found some updates on clients to cause old USB dongle keys to fail after a random period of time.
Even though the key is perfectly valid and readable with the Update key App.

I can confirm that some old USB dongles could become faulty. Back in the days there was a faulty batch of dongles that could have been replaced if you contacted Support. I still have a small utility that could identify these faulty keys from an internal ID number that is not visible within the standard Citect USB dongle utilities.

The issue was also visible on the dongle itself as the LED went off when it occurred. Re-inserting the dongle would revive it again for the time being, which could range from a few seconds to hours or even days.

HARDWARE KEY

An easy check before with the dongle is to see if it is recognized with the CiuSafe software.
You can also try to reactive the key using this software.
At the end with all your findings, make sure to contact Aveva Support and I will make sure to provide the USB dongle number.

We used to have an issue where the USB port was "allowed to be turned off to save power after xx minutes"
Not sure if the latest driver has addressed this, or if this depends on the operating system.