[RESOLVED]: This issue was resolved while I was typing this message, but I'll post it anyway just for your reference.
I auto-updated 5 plugins. Everything updated fine except for Snapshot. Tried again after about 15 minutes, and then it update just fine.
Note: I always keep the latest version of WP installed. Today there was an update for WP (3.8), and I tried updating these plugins prior to upgrading, 4 of which updated fine, and also after upgrading. Snapshot was the only one that gave the error. [RESOLVED: I waited another 10 to 15 minutes and the auto-update worked.]
-- error message --
Unpacking the update…
The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
-- end of error message --