Might I ask that you try again, in case it was a temporary issue on the server?
If you still have the problem, can you check the installer troubleshooting page (which for the site you shared would be at hidden link) and see whether it gives details of any issues?
Some server configurations or security plugins can block external communication or AJAX requests.
I recommend installing a fresh copy of the Toolset plugins, downloaded from the Toolset Downloads page ( https://toolset.com/account/downloads/ ) and see if that resolves this matter.
If it persists, you're welcome to share temporary admin login details in reply to this message for further investigation.
Note: Your next reply will be private and making a complete backup copy is recommended before sharing the access details.
Simply because the server only allows me to upload and update plugins via ftp
Indeed in the plugin screen (after activating the key on wp config), I had this text:
Downloading is not possible because WordPress cannot write into the plugins folder. How to fix.