I had the same issue connecting to a Centos 8 server running Xfce. Clearly the connection was still responding, as screen elements would resize with the window but mouse and keyboard input would appear unresponsive.
Suspecting the screensaver, I had previously tried disabling all screensaver / screen blanking / power options in Xfce I could find, but this did not seem to help.
Manually testing the screensaver from the options worked fine.

I tried Pavel's workaround of killing the xfce4-screensaver process and that seemed to work (thank you Pavel!).

When checking the list of running processes I did notice that both gsd-screensaver-proxy and xfce4-screensaver were running (makes sense as Centos 8 comes with Gnome 3 installed by default, although it does not work with X2Go, hence why I installed Xfce in the first place!). While killing the xfce4-screensaver process does work, I believe the solution below will work permanently, at the cost of seeing no screensaver at all:

1. Disable the gnome screensaver:
gsettings get org.gnome.desktop.screensaver idle-activation-enabled

2. Kill the gsd-screensaver-proxy process - normally it will automatically spawn a new process if you kill it, if it does not re-spawn then the change above has worked.

It is to be noted that even if you have the Xfce screensaver activated, power options switched on, etc. the screensaver will not appear at all once the above has been done, at least in the simple testing I did.

I suspect it relates to X2go not supporting Mutter, and gsd-screensaver-proxy trying to start it up, but it would need a developer to comment really.

Thanks and kind regards,

On Wed, 3 Jun 2020 07:48:38 +0000 "Pavel Duda - pavel.duda@cz.ibm.com" <pavel.duda@cz.ibm.com> wrote:
> I have found that the issue seems to be the screen saver. Looks like
> screen saver starts up but doesn't show anything on the screen (does
> not blank the screen or starts the so user still see the workspace, the
> keyboard and mouse is "locked" of course. My  workaround so far is to
> kill xfce-screensaver completely to resume control over session.
> Pavel