Some custom names, like ".", "/", "@", and " " will return "The entity UUID provided is in an invalid format" even though is it possible to have entities with those names. To replicate you can type "/testfor @e[name=@]".
Linked issues
duplicates 1
Comments 5
It doesn't matter. The point is that the argument thinks they are invalid names when they actually aren't.
try using '@e[name="@"]' instead of "@e[name=@]", I don't know if this will help but it's worth trying I guess.
Please give the command you're trying to use.