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I found a similar error while decripting a small text message.
Try to change SimetricAlgorithm.cs line 207
from

if (((inputCount %
BlockSizeByte) != 0) && ((algo.Mode == CipherMode.CFB) ||
(algo.Mode == CipherMode.OpenPGP_CFB))) {

to

if ((((inputCount % BlockSizeByte) != 0) && (algo.Mode ==
CipherMode.CFB)) || (algo.Mode ==
CipherMode.OpenPGP_CFB)) {