Tonight I've been trying to work out what this could not reproduce or track anything down.
Does it happen for Comcode tags too? I don't have access on your site to add a block, but I tried adding a Comcode tag and it worked.
Are you able to open up the browser console (in Chrome it's View->Developer->JavaScript Console) and see any error coming up when you click to finish adding the block?
I've tried a fresh install of 9.0.13 and was still not able to reproduce. I believe there could have been a bug like this a long time ago, but I'm not sure. It could be caused by an overridden Javascript template masking a code fix. Or, some difference in server environment.
If you want to provide admin access to login to your site so I can test directly, please open a bug report support ticket.
Tonight I've been trying to work out what this could not reproduce or track anything down.
Does it happen for Comcode tags too? I don't have access on your site to add a block, but I tried adding a Comcode tag and it worked.
Are you able to open up the browser console (in Chrome it's View->Developer->JavaScript Console) and see any error coming up when you click to finish adding the block?
If you want to provide admin access to login to your site so I can test directly, please open a bug report support ticket.