Unable to save workflow after leaving knime standby for long

Hi everyone,
I humbly request for your advice on below issue I’m facing.
I’ve been using 5.11.
-When work on the flow for some time (like more than40mn) and leave the screen standby or close the laptop (not turn off), I found out I can’t save workflow I’m working on.
-Ctrl+S won’t do anything. when I close the flow, it asks me to save or not. but clicking Yes won’t do anything. Then I clicked “No.
-After reopening, I found this error message and my flow has some nodes disappear.

This has cause me problems since what have been building for hours may go to nothing.
when reopen, I sometimes don’t remember which nodes disappear and need to spend time tracking back to find the missing node

I change to use 5.12 and still face this issue.
Note: I use company laptop.
can anyone help, please?