akonadi 4:15.08.2-0ubuntu1 fails to read updates from IMAP server, reporting "log_kimap: Socket timeout!"

After recently upgrading from Kubuntu 14.04 Trusty to 15.10 Willy, the akonadi-server is no longer able to sync the local store (MySql-hosted) to the IMAP server, for certain IMAP server folders reporting an error message like this:

log_imapresource: Starting retrieval for "{folder_name_omitted_for_privacy_reasons}"
log_imapresource: Starting message retrieval. Elapsed(ms): 4112
log_imapresource: MessageCount: 1975 Local message count: 0
log_imapresource: UidNext: 1979 Local UidNext: 1979
log_imapresource: HighestModSeq: 0 Local HighestModSeq: 0
log_imapresource: Detected inconsistency in local cache, we're missing some messages. Server: 1975 Local: 0
log_imapresource: Refetching complete mailbox.
akonadiagentbase_log: 1975
1975
log_imapresource: Fetching 1 intervals
Received: 1 In total: 1 Wanted: 1975
Received: 1 In total: 2 Wanted: 1975
Received: 1 In total: 3 Wanted: 1975
[...]
Received: 1 In total: 44 Wanted: 1975
Received: 1 In total: 45 Wanted: 1975

<== and here it hangs for a while and then it reports the error seen below

log_kimap: Socket timeout!
log_kimap: close
log_imapresource: online= true
log_imapresource: online= false
log_imapresource:
log_imapresource: Cancel task: "killed"
The item sync is being rolled-back.
log_imapresource: online= true
log_imapresource: Fetch job failed "Connection to server lost."
Pass a valid window to KWallet::Wallet::openWallet().
log_kimap: KIMAP::LoginJob(0x100c4d0)
log_kimap: connectToHost "localhost" 1143
finished
log_imapresource: Cancelling this request. Probably there is no connection.
log_kimap: close

...And after a while the cycle repeats itself - akonadi will issue another IMAP login command and re-try from the beginning to sync itself up with the IMAP server - trying to read 1975 message items and bailing out at item #45.

The IMAP server is a DavMail java process running locally, and acting as a bridge to a corporate Microsoft Exchange server.

I can determinstically reproduce the bug by right-clicking the root of the e-mail folder for my IMAP account in the KDE Mail's left-side folder view and selecting the "Update this folder and all its subfolders" option.

This issue was not occurring with the akonadi version that came up with Kubuntu 14.10 Trusty.