In FIPS-only mode, only FIPS-validated cryptographic algorithms provided by Windows CryptoAPI are used. The same applies when Rebex.Security.Cryptography.CryptoHelper.UseFipsAlgorithmsOnly is set to true (this is the default value for Windows running in FIPS mode). In practice, this mostly affects AES/CTR cipher which is not supported by Windows CryptoAPI and therefore not available in FIPS-only mode of Rebex components. If the server is configured to support CTR ciphers but not CBC ciphers, this will result in the error you encountered.
There are several possible solutions to this:
Enable AES/CBC ciphers at the server.
Disable FIPS-only mode at the client by setting
Rebex.Security.Cryptography.CryptoHelper.UseFipsAlgorithmsOnlytofalse.Try using a beta of Rebex Components 2018 R1 that supports AES/GCM ciphers in FIPS-only mode. If the server supports AES/GCM as well, this would solve the issue. I sent a download link to your email address.