Is this possibly do to the W10 update? I haven't changed any of my security apps. Any suggestions/theories appreciated!
Yes, this has happened to me in Windows 10 with Thunderbird in particular. It happens when Edge seems to randomly "take over" the default settings for opening links, .htm, etc. Which browser are you expecting the links to open within? For me, it was Chromium. So I had to manually go through the registry and modify approximately 4-6 different keys. You may be able to re-apply the Default App setting for browser and it may make those changes for you. But it is less to do with the email client not opening links and more to do with default browser settings. One thing you could try is within Settings App - System - Default Apps - try switching the default browser away from your preferred default browser, then switch it back to your preferred default browser. Hopefully that will be sufficient and without need to edit the registry. For the record, I've had Edge take over default browser related settings nearly a dozen times now.
Using Fossamail here on W7 and no problems with opening links. Are you using the last release? (25.2.3) It's fresh from less than a week I think.
Thank you for the v headsup. Unfortunately still won't open links. Kind of just grinds away in the bar at the bottom of the Fossa page. I'll search their forum and see if I can rectify it. Copy/paste not the end of the world of course!