Just adding to this with our experience...
As per others, we've been having intermittent issues on and off since December. We use SSO/SAML through our on-prem ADFS.
For my account this morning, I have opened 9 browsers on 3 different devices. 7 log in fine. 2 give the "Uh Oh" message. Yesterday, it was a different combo and tomorrow will be different again.
I have a case open with Schoolcloud support and it is frankly painful. They insist that this isn't a wider issue with the developers stating that any issues must be localised. However, this thread and others prove otherwise and I've sent the them the links also.
Clearing cookies, incognito tabs or different browsers is our workaround but that's a challenge for some of our staff. We're having to add bookings on behalf of those who can't get in. We may have to revert to username/password auth but that seems a backwards step. We're Parents Evening System customers too with exactly the same auth mechanism and that is working fine.
Renewal is in March so I'm looking to move elsewhere as support is dreadful since the TES takeover.