paolo11 wrote: I am playing against a run heavy offence and want to set-up a 4-4-3 formation, so only 2CBs and a Safety. It shows as an illegal formation although I have played this formation before. Why?

Check that you are not repeating players or have more than one player per position in the defensive line, outside LB and CB... You should only be able to play 2 MLB

1.2. LEGAL DEFENSIVE FORMATION

Defensive formation is seen as "legal" when:

a) It has all 11 players assigned
b) Has at least 2 men on the line of scrimmage
c) Does not have more than 1 nose tackle, tackle (for each side), end (for each side), outside linebacker (for each side), strong safety (for each available position) and free safety (for each available position).
d) Does not have more than 4 middle linebackers
e) Does not have more than 2 cornerbacks (for each side)