I found myself unable to place Chlorine into an IC2 chemical reactor to make titanium tetrachloride (probably erroneous info on the recipe). I found the chem reactor would not accept it, and I had to remove mods one-by-one until I got it to work again. Turns out, removing mekanism fixed the problem. Mekanism also prevented me from inserting oxygen into my gregtech electric blast furnace. I don't know why, but I've had to remove mekanism till a fix is found! Letting you know. Thanks!

This is not a Mekanism issue- it is likely IC2 or GregTech are improperly interfacing with FluidRegistry.