IMO these conditions:
he/she added you and you blocked him at the end of the conversation don't affect on how the encryption works.
The most possible cause is that you mispelled the password.
As the answer of your question, the answer is simply no. Sorry...
How about the other logs? Can you open them?
If you haven't changed your password, make another conversation (with anyone) and see if you can open the log.
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