Sort by Product Modified Date bug

[VMRESLO]

Hello everyone!

I have noticed a problem with sorting by Product Modified Date. When I sort my products by Product Modified Date (translated into Razvrsti po: and then Datum objave) on my webpage (http://drazba-drazbe.com/nepremicnine) the sorting mechanism works perfectly, but if I try to sort again after sorting by Product Modified Date, other sorting mechanisms get all wrong and twisted. What happens is that after sorting by Product Modified Date all other sorting is no longer working as it should. Other sorting mechanisms work perfectly, regardless the order in which I select them. Only sort by Product Modified Date causes issues.

I am using VirtueMart 3.0.6.2 on Joomla 3.4.1 with PHP 5.5.

Please help and thank you in advance!
Best regards!