Error message to say can't install add-ins (where ones I have stop appearing on toolbar)

Hi,

I have an accountancy add-in for Excel which normally appears on my toolbar when I load Excel up. For some reason it wasn't there all of a sudden so tried to load it again as an add-in but just get an error message at the bottom of the screen to say that it can't be loaded and to try again later. Tried chatting with Microsoft Support who were no help at all and tried to blame it on the third party software not Excel until I tried loading other add-ins (including Microsoft ones) and get exactly the same message.

Tried logging in and out of my account and shutting down and re-starting but to no avail.

Any ideas/solutions would be most welcome!

Thanks,

David