Overnight problem: The default theme's directory is wrong, please ..."

[SMiFFER]

This literally came over night. Nothing that I changed:
"The default theme's directory is wrong, please correct it by clicking this text"

I also tried to fix this by clicking "attempt to reset all themes" at the bottom of the page admin -> themes and layout -> theme settings, but that did not solve it either.

What really gives me a headache:
Who or what changed that overnight?
Is this a severe issue?
How else can I fix it?

[Aleksi "Lex" Kilpinen]

If I had to guess, your host changed something that affected the server folder path. You should be able to compare the folder structure in your hosts filemanager and SMF settings and see if they are different.