Testnet running v0.12.0.45-b2b2cef win32. MN wallets were updated and synced, then shut down for a few hours. When restarted, wallets got to the current block height then got the message "Synchronization failed". These are the last line in one of the debug logs:
"2015-08-17 12:39:56 ProcessNewBlock : ACCEPTED
2015-08-17 12:39:56 UpdateTip: new best=00000042cfa50dad4de35e58107a9a6b0fb34572109be4481b8c25f5e4c87da4 height=85668 log2_work=43.557896 tx=116660 date=2015-08-17 12:39:39 progress=0.999999 cache=346
2015-08-17 12:39:56 ProcessNewBlock : ACCEPTED
2015-08-17 12:39:59 CMasternodeSync:
rocess - ERROR - Sync has failed, will retry later
2015-08-17 12:40:09 CMasternodeMan::AskForMN - Asking node for missing entry, vin: CTxIn(COutPoint(66eb54fc004c69c3ee0c9688a9f0910c3b5504565bda9094b7a08a5747ff5510, 0), scriptSig=)
2015-08-17 12:40:09 mnb - Got NEW Masternode entry - 2b732c9fbaf5134f9828cc85b10bc1bca75417bde1ee7be5ea329202030017b5 - 107.170.146.58:19999 - CTxIn(COutPoint(66eb54fc004c69c3ee0c9688a9f0910c3b5504565bda9094b7a08a5747ff5510, 0), scriptSig=) - 1439518015 "
EDIT: On the other hand, another MN wallet in the same scenario just got synced through fine, no problem.....
EDIT: Deleting peers.dat and mncache.dat fixed the sync problem.