This is an old thread (but still the best search match for the error
message).
Several other people pointed at NFS, so it got me to look.
Indeed what appear to happen (for me at least) was
The folders being imapped, some of them were on a NFSv3 mount. The NFS
server had been shutdown.
dovecot 2.1.x and 2.2.x has "just worked" with a minimal configuration
for just about anything I've ever thrown at it.
Now, with 2.3.0, there is one user (and only one user) that continually
sees horrible delays, which is not surprising given the logs:
Error: imap(user)<70175><356C8oljxrCBUppP>:
