There could be a number of legitimate reasons for this. Number one IB debits/credits cash based upon settled funds not the closing cash balance. Second, you may have an overall credit balance of settled funds but it may be the net of long and short balances in various currencies. Short currency balances are esssentially a loan and are subject to interest charges (interest debits and credits are calculated on a per currency basis as rates vary by currency type). In addition you may be long settled cash in the security sub-account, and short a lesser amount in your commodities sub-account. Interest calculations are performed separately for each of these balances. Finally, if your net long cash balance includes proceeds from short stock sales, such balances are subject to a different interest rate calculation and are therefore separted from remaining cash (which may represent a debit balance). If you send a webticket to IB Customer Service, they will breakout the calculation for any given day.
is your base currency EUR and you trade US shares or futures? or USD against EUR? If yes, thats the reason
I usually have a small amount of short interest paid each month that shows as a negative interest while on the same statement I will receive positive interest on the overall positive cash balance. Not a problem when holding short positions.
what happened is I had a few shorts that got very expensive to carry, and that wiped out any positive earn I had. any shorts you have , better check and see how much its costing you. though with the market going to zero, thats not a real problem.
I've also been getting negative interest for the past two months. It was a shock to go from +.15%/month to minus. I just assumed it was due to reduced interest on free credit balance and increased short costs.