An IT failure at Lufthansa stranded thousands of passengers and forced flights to Germany’s busiest airport to be canceled or diverted Wednesday, with the airline blaming botched railway engineering works that damaged broadband cables.

More than 200 flights were canceled in Frankfurt, a vital international transit hub and one of Europe’s biggest airports, a spokesperson for operator Fraport said.

Lufthansa later said its IT systems were rebooting and that flight departures had resumed from Frankfurt. The carrier expects the situation to stabilize by Wednesday evening.

Scores of flights were also delayed, data from FlightAware showed. Photos and videos from several German airports showed thousands of passengers waiting to be checked in.

“We wanted to go to the wizard convention in England, in Blackpool. And now we are stranded here,” Alexander Straub said at Frankfurt Airport. “We have eaten some pretzels and are still waiting,” said his fellow passenger Marc Weidel.

Lufthansa and Germany’s national train operator blamed the problem on third-party engineering works on a railway line extension that took place Tuesday evening, when a drill cut through a Deutsche Telekom fiber optic cable bundle.

That caused passenger check-in and boarding systems at Lufthansa to seize up Wednesday morning and prompted German air traffic control to suspend incoming flights, though these have since resumed.

Other airports also reported cancellations as a knock-on effect. Paris' Charles de Gaulle Airport said two flights had been axed and a further two flights had to turn back around.

Amsterdam’s Schiphol Airport, among the world’s busiest, reported one cancellation of a flight to Frankfurt.

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Pen and paper

Shares in Lufthansa, which also owns SWISS, Austrian Airlines, Brussels Airlines and Eurowings, recovered earlier losses and were up 0.2% as of 1518 GMT, while Fraport shares were down 0.2%.

Passengers said on social media the company was using pen and paper to organize flight boarding, and that it was unable to digitally process passengers’ luggage.

In a tweet, Lufthansa said: “As of this morning the airlines of the Lufthansa Group are affected by an IT outage, caused by construction work in the Frankfurt region.”

Deutsche Telekom said in a statement: “Two cables have already been repaired overnight by our technical team and many customers are already back online”, adding that the situation was improving continuously.

Deutsche Bahn apologized to Lufthansa passengers for the inconvenience caused.

The IT system failure comes two days ahead of planned strikes at seven German airports that are expected to lead to major disruptions, including potentially to the Munich Security Conference where world leaders are expected to gather.

Scandinavian airline SAS said it was hit by a cyber attack Tuesday evening and urged customers to refrain from using its app, but later said it had fixed the problem.

Unknown attackers cut cables belonging to Germany’s public railway in December in what was seen as a second act of sabotage against Deutsche Bahn in as many months.