Ionospheric calibration is as yet untested with the EVN, but in observations using the VLBA, it has resulted in the calibration of 50-80% of the phase effects induced by propagation through the ionosphere. Variations in the phase due to the ionosphere can severely affect observations, even those using phase-referencing with nearby calibrators and fast switching times (under some conditions). If you use phase-referencing at L-band or even at higher frequencies, where problems can still arise, it might be worth running the task TECOR. This attempts to correct for the ionosphere by using maps of vertical total electron content (vTEC) to derive the dispersive delay.
The ionospheric maps need to be downloaded from NASA's Crustal Dynamics Data Interchange System (CDDIS) and are available from 1998, day 151. See EXPLAIN TECOR for the latest download instructions and a discussion on which file to download. The corrections should be done before fringe-fitting.
If the observations go across midnight then you will need to download maps for both days and set INFILE appropriately. The solutions should be checked in SNPLT with OPTYPE 'DDLY'.