I have Lightroom 5 on my Macbook Pro (10.11.6), and I have been able to watermark just fine, until a month ago when it locks up each time I export with a watermark. I have to restart the program to use Lightroom again.
I'm thinking I should unistall and reinstall the program - will I lose my catalogues if I do this?
No, you will not loose anything at all. Please restart you machine before you reinstall the Lightroom 5 again on your machine.
A month ago did you create a new Watermark using a different font then you had been originally?
The Watermark system in LR doesn't like some fonts.
Indeed I did, that could very well be the culprit! Thanks.