dovecot - Oct 2014 - Proxy problem: "[COMPRESSIONACTIVE] TLS compression already enabled"

We're proxying using 2.2.14~rc1 (on our IMAP Proxy) to two dovecot
backend servers running dovecot-2.2.13-r1

When we're using Thundebird to connect to the dovecot proxy, we're
getting the message "The mail server for account ACCOUNTNAME
responded: [COMPRESSIONACTIVE] TLS compression already enabled"

But why?

Capabilities reported by the proxy:

. OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE
IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS
THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT
CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC
ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS SPECIAL-USE
BINARY MOVE SEARCH=FUZZY COMPRESS=DEFLATE QUOTA] Logged in


Capabilities of the backend servers when connecting directly:

. OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE
IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS
THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT
CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC
ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS SPECIAL-USE
BINARY MOVE SEARCH=FUZZY COMPRESS=DEFLATE QUOTA] Logged in

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On 14 Oct 2014, at 15:24, Ralf Hildebrandt <r at sys4.de> wrote:
> We're proxying using 2.2.14~rc1 (on our IMAP Proxy) to two dovecot
> backend servers running dovecot-2.2.13-r1
> 
> When we're using Thundebird to connect to the dovecot proxy, we're
> getting the message "The mail server for account ACCOUNTNAME
> responded: [COMPRESSIONACTIVE] TLS compression already enabled"
> 
> But why?
Is the connection from proxy to backend using SSL? Did this work in earlier
Dovecot version?