Encryption of SignalMessage fails 'n' times then succeeds with the same message

[timt]

I have a strange situation where after having successfully processed/decrypted the PreSignalMessage, calling SignalSessionCipher.encryptData fails with a 'Code 0 - Unknown error'

After some investigation, I found that if simply repeating the call to SignalSessionCipher.encryptData step recursively it will eventually succeed.

Can anyone shed some light on this or point me in the direction of how to isolate the problem?