1) Change your tactics slightly.
2) Train fitness for two weeks in pre season with tactics match preparation then two weeks tactics with team cohesion match preparation then about a week or two (depending on amount of new players in) team cohesion with attacking match prep.
3) When season starts forget about teamwork (unless while watching matches you see players giving ball away too much etc/ you have more than 5 new players whereby have one week more of it). Then use team training according to your philosophy (ie attacking) and match preparation depending on your opponent. But if you are conceding too many alternate between defensive positioning and defending set pieces for a couple of weeks and see if it makes any difference.
4) Put one player in front of your back four as an anchorman and see if that makes a difference.
5) If you concede from crosses look at your fullbacks and perhaps have them to "defend" rather than automatic but be wary that that may impact on your attacking play.
6) On the 40th min till half time and 85th till end shout instructions contain, drop deep, play even safer, play narrower.
Alternatively with PSG you may try to outscore every opponent!