Ok, we have sorted out the issue with the withdrawals and I will process them in the next couple days. What had happened was that the old addresses (before I started managing bitcoinpyramid) were still present in arsenische's bitcoin wallet and some of the deposits were spent there.

We identified which were these transactions and also removed this wallet so it shouldn't happen again. Eventually I will need to figure out a way to restart the database as we have very few active members and many addresses' private keys have been lost by the owners. Right now I am thinking about a way to validate active addresses (for example asking for a signed message from the address) and hold the funds for addresses that aren't validated. I would appreciate any thoughts or suggestions about this.