Error in calculate taxes in categories new product

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Error:can't calculate taxes in admin/categories when i try to add a new product

I have two tax classes, when i select the first one the net and gross price is the same, and when i select the second tax class the system calculate double tax from the correct.

 

eg

tax class --products --(tax 18%)

Products Price net = 100

Products Price gross = 100

 

and

tax class --service-- (tax 18%)

Products Price net = 100

Products Price Gross = 131

 

 

Any Idea?????

Thanks