Hi Krishna,
It seems like FWS is failing to decode the base64 encoded SAMLRequest. Hence you are getting below error in server.log
[ERROR] - Exception while decoding base64 string: com.rsa.jsafe.JSAFE_InputException: Unexpected padding chars
There could be many reasons to get this error.
1) SAMLRequest is not encoded properly or not base64 encoded
2) SAMLRequest would have modified by custom login page or appended with some junk characters
3) half the request might have encoded and half of the request would be in clear text format.
Thanks,
Sharan