Hi, I have a problem I just can't deal with. I have uploaded designs that are brown with texture. The plugin changes them to black for me. How to get rid of it?

It might be a technical issue then, please open a support ticket to get technical support.

https://support.fancyproductdesigner.com/support/tickets/new

It's not transparet, I tried converting it to jpg, removing the white and resaving it as a png. The result is the same.

Please make sure that the texture area should not be transparent.  At the moment, it seems you are using a PNG format image with that texture area transparent.