From gnupg-w32-2.2.30 above and gnupg-w32-2.3.2 above, gpg cannot open gpg-agent. The error message is like:
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>gpg -d password.txt.gpg
gpg: encrypted with elg2048 key, ID AABE0D7867BFA077, created 2008-03-25
"Lu Wei <luweitest@gmail.com>"
gpg: can't connect to the gpg-agent: Filename too long
gpg: can't connect to the gpg-agent: Filename too long
gpg: keydb_search failed: No agent running
gpg: public key decryption failed: No secret key
gpg: decryption failed: No secret key
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"Filename too long" is confusing since that's not true. Test environment is Windows XP SP3.
Maybe it's another WinXP specific problem; but if it could be resolved easily, please take a look, and I am ready to help testing.