I am using

CodePudding user response：

Your key size is 1024 bit:

# key.pem is
# -----BEGIN RSA PRIVATE KEY-----
# MIICXAIBAAKBgQC8kGa1p...
# ...
openssl rsa -text -noout -in key.pem

RSA Private-Key: (1024 bit, 2 primes)

Now let's check for minimum requirements for RS256:

Required key size: At least 2048 bits

You have to create more secured key