TLDR: You are trying to send too many MRT at once. Send MRT in more transactions divided in smaller chunks.
This error occurs when your wallet balance is made up of too many small transactions. It’s like having a bag full of change instead of larger bills. When you try and send a large amount of MRT that’s made up of too much change, it clogs the transaction because it became too big. This is very common, especially if you’re pool mining.
Solution to this situation is to send desired amount in smaller pieces. If you tried to send 100 and this error appeared, try to send 50, if it still appears try 25 and so on. Once you're successful with sending, you can send another transaction. In general, you can "clear" your wallet every day by sending whole amount to your own wallet. This way you will be ready to send even bigger amounts of merit at once. Sometimes it's required e.g. when posting sell order on market or trading OTC to save time during trade.
Don't be confused of non-specified "Internal server error", usually it means the same error as "TX_MAX_SIZE_EXCEEDED" described here.