Was the "Air Berlin" lap of honor over Düsseldorf problematic?
The maneuver was safe.
A spokesperson of the DFS (German Air Traffic Control) told the Berliner Morgenpost that the pilot requested and was given clearance to fly a left curve in case of a go-around. The necessary distance to other aircraft was ensured and a specific height for the maneuver was not determined.
The spokesperson also said that the DFS believes that this maneuver was safe.
However the LBA (Federal Aviation Office) started examining the case and requested Air Berlin to submit a statement which provides a reason for the go-around. The editorial staff of that newspaper believes this to be a routine procedure.
source (in German)
A rough translation of the relevant part in english:

Christian Hoppe, spokesperson of the German Air Traffic Control confirmed that: &quot;The pilot previously requested if he is allowed to fly a left-curve in case of a go-around and was given clearance.&quot; The necessary distance to other aircraft was ensured, the specific height of the maneuver was not determined. &quot;From the DSF's point of view the maneuver was safe.&quot; The Federal Office for Air Navigation corrected earlier information, which claimed that the pilot had reported problems with the landing-gear. &quot;That was our assumption.&quot;
The Federal Aviation Office will nonetheless examine the case, Air Berlin should submit a statement on the reason of the go-around. Our editorial staff believes, after multiple requests, that this is most likely routine operation.