We used a HPLC method for years, SCX column, mobile phase: ACN:Buffer 60:40 (buffer is Na2SO4 14 mM) at 2ml/min, 1800psi (50oC). The only thing changed is that we add now 50 µl of Krebs buffer in our 0.250 ml of plasma to be extracted, on PH SPE cartridge. Is it possible that this is the cause of an increasing pressure at 2500 psi? We aren't able to decrease this back-pressure, by several cleaning methods.

Salts can be washed out easily with water, if you are lucky enough to have only a partial obstruction due to precipitation. Probably, you have the biopolymer problem discussed in the chain "frits versus sieves". Plasma has enough variation to account for your observed change. Also, could it be that your Krebs buffer interferes with your extraction?
Incidentally, you use only Na2SO4 in your aqu. part, no real buffer? The Na2SO4 at your conc. should be chaotropic, so that´s ok, but no pH buffering?