I am having the same issue since last Tuesday. I have an somfy Connexxoon. I contacted Somfy support, but they were not very helpful. They just stated that the Connexxoon can only be controlled via the Connexxoon phone app. I can login in the connexxoon app and the somfy connect website. The tahoma website doesn’t work for me either. I can log in, but then shows me ‘An error has occured on the server’. When I purposely use a wrong password it shows me the wrong password error. So I don’t know what that first error message means.
I tried a new install in docker, to check if its something in my configuration. But didn’t work. Then I installed Home Assistant on my Synology Docker. I think they use a different method of authentication, since I didn’t need to enter a password, but got a prompt from the Somfy website to allow access. Then it worked.