unfortunatelly i dont remember if i had the problem to the extend you describe and i have no clue how i fixed it if it even was as bad as in your situation.
Depending on your PRTG-Version is would suggest you try to update your PRTG or just restore a backup from a few days ago when everything was fine.
Just in case you or someone else might need this, here is the workaround Florian Lesage talked about:
"In the current PRTG version 21.2.67, object types in the Properties section of the Map Designer that are by design incompatible to the selected object in the Device Tree section return an error dialog window when loading the object preview. The message is Error (Bad Request). Sorry, the selected object cannot be used here. PRTG redirects you to the welcome page when you confirm the dialog window.
This happens, for example, when you select nothing, root, or a probe, group, or device in the device tree and hover your mouse over a live graph or gauge object in the properties section. Note that existing maps that already include an element that is empty due to incompatible object type will show the same error message and will not load.
Workaround: To add a live graph or gauge for a sensor, ensure you have selected the sensor in the device tree section (click the sensor in the device tree section to select it) before you choose an object in the properties section. If you have selected any other object in the device tree section than a sensor, avoid hovering incompatible object types like live graphs or gauges in the properties section.
Status: Resolved with PRTG version 220.127.116.112."