I have suddenly started getting problems. It's most unusual for me to have problems with ARV! Has there been an automatic update?

The biggest problem is pen lag, huge pen lag. Turning off 'multithreading' has solved that but I've never had to do that before.
Some shortcuts have stopped working. E.g. Shift-Cmd-C just says that there is "No paint on the layer" but using the clear command from the layers panel works perfectly.

Any ideas?