Berlin Alexanderplatz station's ticket office is open every day. However I've noticed they seem to be understaffed at times with long queues.
It's close to my home so I drop by every so often but staff here often isn't exactly well trained. Unusual wishes (for example, splitting a DB gift certificate into smaller denominations, or low value credit card transactions) were supposedly "impossible" for staff to carry out. But these wishes were duly fulfilled at Berlin Ostbahnhof's ticket office.
(What did upset me though was wrong information given to me a few years ago. A certain type of expensive ticket was supposedly not eligible for frequent traveller points as it was not BahnCard discounted. In fact it was eligible - if I had known that this was untrue, I would have bought it elsewhere. Deutsche Bahn has a policy of not crediting points for past journeys if they have not been claimed at the point of purchase.)
No first class/frequent traveller counter available, unlike at Ostbahnhof, Hauptbahnhof or Zoo. read more