Answering my own question here (others may find it helpful). I followed @Kevin-Monahan suggestion for another problem I was having. That is, I tried resetting the preferences upon startup.
It actually worked. I can paste text again!
This success has come with a number of other problems, not least that Premiere is running infinitely more slowly now. I'd screenshot all the different panels in the Preferences dialogue box so that I could get back to my usual settings, but there were other things that changed in the reset, which I was not expecting:
- proxy view switches off. Make sure you turn it back on.
- workspace preferences will need to be rebuilt
- starting the program goes into the 'Learning' mode and workspace, and the list of recent projects you've opened is no longer there.
- and keyboard shortcuts has reverted to default. If you're planning on doing this, make sure you save your custom keyboard layout!