Try reinstalling both the OS and FW.
Just remember the OS is installed at boot and the FW via the upgrade menu in the normal way.
Will do
tomorrow asap and come back here.
Edit:
- Reinstalled OS SDS2000X_Plus_OSV5.4.0_EN
- Tried Webaccess: It works fine now.

- For good measure reinstalled FW SDS2000X-Plus_V1.6.2R5_EN
- Tried Webacess: Webserver is accessible, but e.g.
instrument controls stays empty
- Reinstalled OS SDS2000X_Plus_OSV5.4.0_EN again
- Tried Webaccess: It works fine again.

Thank you Rob! Excellent support as always! Good to have you here!