The symptom here is a tiny bit better:

nov 16 14:50:22 marcos dovecot[3520770]: imap(anarcat)<3615127><L3LtO+3Q4tXAqAB1>: Error: Mailbox Sent: Maildir filename has wrong W value, renamed the file from /home/anarcat/Maildir/.Sent/cur/1454871880.M36279P8532.marcos,S=736,W=792:2,S to /home/anarcat/Maildir/.Sent/cur/1454871880.M36279P8532.marcos,S=736:2,Scache record uid=38388: UID 38388: Broken virtual size in mailbox Sent: read(/home/anarcat/Maildir/.Sent/cur/1454871880.M36279P8532.marcos,S=736,W=792:2,S): FETCH BODY[] got too little data: 754 vs 792=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=2 body_bytes=215414:50:23 marcos dovecot[3520770]: imap-login: Login: user=<anarcat>, method=PLAIN, rip=192.168.0.117, lip=192.168.0.3, mpid=3615137, TLS, session=<NWF4PO3Q5NXAqAB1>

ie. Dovecot recovers, but this breaks the connection on mbsync's end,
which means it takes a loong time to recover.