I've got dirmngr 2.1.3 configured to talk to hkps://keys.mayfirst.org, using the
mfpl ca [0].

I can refresh single keys, or even a match of several dozen keys.

But when i try to refresh a keyring with hundreds or thousands of keys, dirmngr
chokes.

I first reported this in the following thread from 2.1.1:

  http://lists.gnupg.org/pipermail/gnupg-devel/2014-December/029214.html

After about a hundred keys get processed, gpg --refresh terminates with:

gpg: Total number processed: 83
gpg: unchanged: 83
gpg: keyserver refresh failed: Input/output error

I do *not* have this problem with cleartext hkp, only with hkps.

[0]
https://support.mayfirst.org/raw-attachment/wiki/faq/security/mfpl-certificate-authority/mfpl.crt