An "unusually high number" of pilots called in sick in what seems like a strike against Air Berlin who is negotiating the transfer of staff to a potential buyer after suffering years of losses.
After losing £2m per day in 2016, and seeing its financial life-support cut off when Etihad abandoned its investment in the company in April, Air Berlin is in liquidation.
A wildcat walkout by Air Berlin pilots could "threaten the existence" of the under-fire carrier, its chief executive has said.
Passengers booked to travel on the cancelled services as part of a package deal should contact their tour operator, Air Berlin said in a statement.
Flight passengers crowd in front of the desk of German airline Air Berlin at the airport in Duesseldorf, western Germany, on September 12, 2017, after the troubled airline had to cancel flights due to "operational reasons".
"We are now conducting final talks with potential investors".
On Sunday his company Intro offered 50 million euros immediately for the stricken airline, with up to 450 million euros of further payments "depending on performance", he said.
This has the potential to unsettle any potential rescuers for Air Berlin, which filed for administration last month when it was given a €150m (£135m) loan by the German government.
"A stable operation is an essential precondition for successful negotiations", Mr Winkelmann said.
German flagship carrier Lufthansa - which already leases 38 of Air Berlin's 140 planes - could buy up to 70 aircraft with as many as 3,000 crew for Eurowings, German media had reported.