Had this issue a few times too.

The only solution i found was to restart the services of SOTI Assist in the admin tool.

Agentversion: 14.4.0.1028

Plugin: 

Zebra TC51 with Android 8.1

The logs show : 

• requested ;
• started;
• terminated

For your reported problem with html5 remote control session, try closing ALL browser sessions (including the MobiControl web-console and other browsing sessions) on the machine you previously used to start an html5 remote control session.  Then restart your browser, log in your MobiControl session and retry staring your html5 remote control session on your device of interest.

I'll often send a restartagent script to the device in question to force it to refresh if I know that it was my RC session that caused it to hang. This can be disruptive to the end user if you are using the lockdown however as that also will be refreshed in the process so use it with caution. 

I do what Matt does. I send a script to it and it will allow me back in.

Mine is set up this way:

restartagent
sleepex 350
connect -f