Public key and public address in Bitcoin and crypto are often used synonymously, even though technically speaking it is not the same.
We opted for the way the majority of people uses these words.
Did you copy your complete signed message, including the "=" at the end?
I am also facing same problem. including " = " too but no success same error.
That is strange. I just tested it with a new address, it worked fine.
Can you give me some details please? Did you sign using QT or coinomi