Based on your screenshot I understand that you are opening an event monitor query in the Modern interface on a new tab. What happens if you open it in the same tab? Can you open the query in the Classic interface without any issues?
I believe the assyst version being used is assyst 26R1. Could you check whether the following system parameters were set to true after the upgrade to 26R1 and the assyst server was then restarted?
Reset Form State
Reset Query Profile State
You do not need to manually set them back to false, as this happens automatically once the restart is complete.
This ensures that, following an upgrade, the interface picks up any system form definition changes between the previous version and the new release. You only need to do this once.
When logging in to the assystWeb interface, please also make sure that you are using a supported browser version and have cleared your browser cache. [ Installation:MSSV - assyst Portals - assyst26R1Wiki]
If this does not resolve the issue, I recommend logging an incident with our Support team in the usual manner so that they can investigate it further: