tah and blue background

I installed tiles@home. After some time I found that every rendered tile has blue background (same as sea). I stopped tah because I wanted to install it on another computer.

Yesterday I installed t@h on another computer. I verified that all ok and no blue background. But today I found that bug appeared again.

Sample: http://shurik.aliki.ru/2601.png (13,4895,2601)

Is this a bug of tah or no?