This has been happening for some days now, and I finally decided it is not a "temp" error. 

There are two things happening (and I am not yet sure if they are related or not):

1. evolution forgets POP passwords. I actually checked the gnome keyring, and the passwords are deleted. Not sure (yet) if this is evo or g-k.

2. when, on a POP dialog, evo gets a different response, it loses track of where it is.

For this second issue, I have this output. Notice the dialogs for password at the beginning.

e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
e-data-server-ui-Message: Unable to find password(s) in keyring (Keyring reports: No matching results)
e-data-server-ui-Message: Key file does not have key 'pop:__xxxx@pop.gmail.com_'
POP3_STREAM_LINE(55): '-ERR [SYS/TEMP] Proxying required, but not implemented.'
POP3_STREAM_WRITE(21):
PASS xxxxxxxx
POP3_STREAM_LINE(79): '+OK [SYS/TEMP] Due to a temporary system problem, this mailbox will seem empty.'
Got + response
CamelException.setv(0x7fb9c4857490, 304, 'Unable to connect to POP server pop.gmail.com.
Error sending username: +OK [SYS/TEMP] Due to a temporary system problem, this mailbox will seem empty.')
POP3_STREAM_WRITE(13):
USER xxxx

POP3_STREAM_LINE(25): '-ERR Command not accepted'
POP3_STREAM_WRITE(21):
PASS xxxxxxxx
POP3_STREAM_LINE(25): '-ERR Command not accepted'
CamelException.setv(0x7fb9c4857490, 304, 'Unable to connect to POP server pop.gmail.com.
Error sending username: -ERR Command not accepted')
POP3_STREAM_WRITE(13):
USER xxxx

POP3_STREAM_LINE(25): '-ERR Command not accepted'
POP3_STREAM_WRITE(21):
PASS xxxxxxxx
POP3_STREAM_LINE(25): '-ERR Command not accepted'
CamelException.setv(0x7fb9c4857490, 304, 'Unable to connect to POP server pop.gmail.com.
Error sending username: -ERR Command not accepted')
CamelException.set(0x7fb9c4857490, 3, 'User canceled operation.')
POP3_STREAM_WRITE(6):
QUIT

POP3_STREAM_LINE(13): '+OK Farewell.'
Got + response