Pilots union Vereinigung Cockpit (VC) said it was surprised by the absences and that it had not called on its members at Air Berlin to call in sick.
It seems like sickness is in the air since their cancelled flights aren't.
The move has also hit Lufthansa subsidiary Eurowings, which said it had been told by Air Berlin at short notice some of the flights the latter carrier runs for it could not be staffed.
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.More news: JJ Abrams to direct 'Star Wars IX': Lucasfilm
"A stable operation is an essential precondition for successful negotiations", Mr Winkelmann said.
After losing the lifeline of regular cash infusions from Gulf carrier Etihad in June, Air Berlin filed for insolvency on August 15.
The airline has long struggled for survival, and booked losses amounting to 1.2 billion euros ($1.4 billion) over the past two years. "That's the only way we can secure as many jobs as possible".
The objective of the loan is to allow Air Berlin to continue operations and maintain its services while it concludes ongoing negotiations to sell its assets.More news: Donald Trump's Caribbean Home May Have Been Badly Damaged By Hurricane Irma
The flight cancellations represent a "massive threat" to Air Berlin's insolvency proceedings, in the view of the carrier's court-appointed executive director Frank Kebekus.
In the meantime, the airline has been kept aloft by a hastily-agreed 150-million-euro loan from the German government.
Air Berlin is Germany's second largest airline and employs over 8,000 staff.
With the deadline drawing close for investors to submit bids for Air Berlin, operations are facing increasing disruption.More news: 'Dancing with the Stars' 25 Cast And Partners Revealed