I am using Joomla V 3.9.25 and SCLogin with extStore popup plugin.
When a user registers, and (presumably) activates their account, following the activation subsequently throws a 403 error instead of the 'you have already activated ... etc.. msg.
Is there a setting to control that?
I have the registration component set to Joomla
Here's the SCLogin View settings page prntscr.com/110l8md
I used to use BTLogin, but have disabled that - could it still be getting in the way? should I actually remove it?
The SCLogin module doesn't control activation whatsoever. It's really just a pretty template around the com_users extension with integration for JFBConnect social authentication.
If you're seeing issues with activation, I'd recommend trying the bog-standard mod_login module and see if you're having the same problems. My assumption is you are and, in that case, it wouldn't be related to the SCLogin module.
Unfortunately, I don't have a good suggestion as to where to look. It seems like users are either getting activated automatically or there's some sort of loop when they go to the activation page that activates them, reloads the page and then errors out because the activation just happened. Not sure how to investigate that much further though, but hopefully that gives you some ideas.