General question regarding software distributed at torproject

Error checking sig of downloaded software from archive.torproject.org

Output sig check:
gpg: using ? key '..numbersandletters..'
gpg: Can't check signature: Invalid public key algorithm

Does anybody know what an 'Invalid public key algorithm' means and how to solve it?
It was not a problem 2 weeks ago and software downloaded 2 weeks ago does not give these new errors. New downloads via other cricuits don't solve the error-problem.