Save button doesn't work anymore

We are facing the following issue: the save button doesn't work anymore (for ALL users, including the admin). If you click on it, it becomes "grey", like "not selectable" and it doesn't save.
This is the error returned in the console: 
Error on loadContent('../tool/saveObject.php?csrfToken='&#44; 'resultDivMain'&#44; 'objectForm'&#44; 'true')<br/>Destination 'resultDivMain' is not a node or not a widget

It is not the first time happening, but last time, after rebooting the machine, it seemed to be solved. This time, even after rebooting the machine, it is still occurring.

Thanks in advance for your help.

Sure, here more information:
* version is v12.0.2 but it was happening also with v11. Then we upgraded and seemed to disappear but after few days it occured again
* Browser: we tried with different ones and it is the same
* Logs: no errors in the log
* It applies to all save buttons
We are available to go for "assistance" and schedule a remote meeting to better show you the issue.
Thanks again,
Silvia

Shall we go for a remote assistance session instead?
I sent an email asking for information but I didn't receive any answer yet. In any case, we would like to have a shared session to understand the root cause and be sure that it won't occur anymore.