After some more inquiries, looks like you were right : this morning drewdrew (part of the team) who I talked to during the last hour found the blockchain being very slow. He didn't think of a problem until a few supporters reported the solo-mining bug. With the help of owlhooter we found the problem :

WHAT HAPPENED
 - there has been a temp fork this morning but the blockchain resynced well in later blocks.
 - the submitted blocks from solo miners were most likely to a pool with an outdated wallet and caused their blocks to disappear therefore their coins too. We hope that by fixing the issue, they will get them back.
 - we asked bittrex to stop the trade in order to fix the blockchain issue.

WHAT TO DO
 - do not make any transactions with H2O until we fix the issue, just in case ;
 - check your block height (hover the little tick on the bottom right of your wallet) and compare it to the one of the network here : http://cryptexplorer.com/chain/H2Ocoin ;
 - mine on pools until issue is solved because only solo miners were affected by the issue ;

WHAT WE'LL DO
 - owlhooter will help us from now on as our dev.
We hope his presence will not be temporary but he seems to be interested and found our project to be "ambitious"  
 - he will put a hard fork in the blockchain issue by implementing DarkGravityWave3 at a future block (not determined yet) in order to fix the issue because H2O is currently using the 1st version. It will be tested on the testnet to be sure then released.
 - if ok, we'll ask the solo miners, pools and everybody to upgrade their wallet (and backing their wallet.dat before doing so  )
 - none of your coins will be lost because the fork will be on a future block.