Pycharm hangs after splash screen but before sub-windows load

I am running PyCharm CE 2020.3 on Red Hat Linux 7.9. I have been using this quite happily for some time, updates were applied to my machine yesterday and now PyCharm won't start. The splash screen displays, runs through and clears, the main IDE window appears with the menus up the top, but none of the sub-windows load. The menus at the top are unresponsive, I cannot close the window with the X button and after a short while (maybe 30s) the window seems to set itself to always on top or something - the terminal window behind remains active and can be used but cannot be brought to the front. It is necessary to kill PyCharm using `kill -9`.

I have tried renaming both the cache and config directories and updating to 2020.3.3, none of these had any effect. No errors are visible in the logs, thread dump shows a load of waiting threads. I've got these zipped but but I don't know how to attach here.