Jenny, I'm sorry there's no fix yet and while I don't like suggesting different browsers (so losing familiarity and access to bookmarks), at the moment that's all I have.
As MaxE said, he also had the same error on Chrome but using Edge fixed it. I know that's likely not available on a phone but Firefox is, probably along with Brave and others.
Mona Pereth wrote:
My guess is that this is an intermittent server-side error, and that you switching browsers didn't really affect it at all, but the apparent fix was just a lucky coincidence.
As Jenny noted, it's not at all intermittent - and it's extremely irritating, preventing me from using Vivaldi on WP: I also re-check often.
Other issues are currently preventing me from using my Linux system - where the same version of Vivaldi works fine with WP.
I take your point about the error code though - it's all being investigated by those involved with the site transfer.
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Giraffe: a ruminant with a view.