Looking at the DB website on past view, it appears there was some form of route closure as the Airport Express is not showing on the planner. Also, it's marked "not 29th May - 31st May" on another bit, so I am guessing there is ongoing disruption. I always try to double check any connection nearer the time if possible, doesn't matter which country. The reason why is because I've been caught out by Northern Rail connections not running in the UK twice (engineering work announced after Advance+connection ticket purchased from Virgin Trains).
For future reference, taking the S-Bahn is quite simple from the Hauptbahnhof - S7 to Ostkreuz and then S9 to the Airport. Doesn't take all that much longer, only about 10-15 minutes more and it's a simple enough journey. I would have thought they could tell you that but oh well.
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EDIT - The
poster here (which is the standard engineering works info for the whole network posted at stations) states that the Airport Express trains (RE7 and RB14) are diverted to Lichtenburg up to 1st June. Services are affected in different ways over about a year, it mentions a booklet. According to the poster, it's part of a series of works to renew two mainline stretches of track and eight bridges. Sadly, it's in German only and I think where they've fallen down is not providing this info in English because it affects airport travellers on this occasion. The RE/RB services are (correctly) not showing on the planner at the moment (and presumably haven't been doing for some time previously). The poster will be at stations, I've seen them attached to the yellow/white timetable boards.