Hi Ivan,
sorry for the trouble.
This can happen in really large tables, with many thousand fields. The browser can then not manage this with the available memory.
My suggestion is to try this again in Google Chrome, which, in my experience, is the fastest browser with this, and which handles this the best.
If that’s also not possible, I’m afraid that your table is too big to be managed on the “Edit” screen of TablePress. You should then export the table to CSV format, open that file in Excel, make your changes there, and re-import the file into TablePress again (replacing the current version).
Regards,
Tobias