I'm getting lower back pain mainly during long climbs (8% >1 mil) and even if I'm pushing hard on the flats for any extended period of time. I did the same routes at the end of last season without any issues.

Is this just a normal part of getting back in shape after minimal riding during the last 8 weeks or could this be due the slight setup change I made (seat up 0.5 cm and increased the seat-handle bar drop from 90 to 110 mm).

I'm getting lower back pain mainly during long climbs (8% >1 mil) and even if I'm pushing hard on the flats for any extended period of time. I did the same routes at the end of last season without any issues.

Is this just a normal part of getting back in shape after minimal riding during the last 8 weeks or could this be due the slight setup change I made (seat up 0.5 cm and increased the seat-handle bar drop from 90 to 110 mm).

wow.

the "slight set-up change" that you refer to is actually a very major change, the 9cm to 11cm is 2cm drop increase plus the saddle up 5mm is a moderate change too. I would question the need for such an agressive saddle to bar drop. You made 2 changes, if you must do it, I think you should do this one change at a time.

First, I would restore the drop back to 90mm and see how you go with just 5mm saddle up.

Lower back pain is a sign the bars are too low/too far or you are sitting too far forward (or all of the above).