I’ve already checked everything you mentioned.
I removed all the plugins for this website and the problem persists.
More than that, when I’m logged in the admin panel, the category which doesn’t have any discounts is shown correctly and when the plugin is deactivated, all the prices are exactly the same as before, but only when I am logged in. If I’m not logged in, the prices are modified as I mentioned above (even if the plugin is not activated).
For you to better understand, I can give you the following example: the first product has a price of 16,99 eur; when I apply a 30% discount, the shown price is 8,66 euro, meaning that the actually applied discount is 51%, instead of 30% as I wanted. When I am not logged in, the price is 14,26 eur and the reduced price is 7 eur even if the applied discount is still 51%. Even if I removed the plugin, the price without any discounts remains 16,99 when logged in, which is correct; but if I am not logged in, the shown price is 14,26, which is not correct.
I didn’t have any other discount plugins installed before. This was the first and only one until now!!