The bomb alert interrupted the celebration organized by the Rothschild bank near Frankfurt on the occasion of the 70th birthday of former German chancellor Gerhard Schroeder.
The Russian ambassador to Germany, Vladimir Grinin, also attended last night's celebration.
Since the hotel where the ceremony was organized received a call threatening a "sinister event", it was evacuated and the police conducted a search, so the guests could only return after 23 pm.
Several German media, including the weekly Spiegel and the daily Bild, announced that the cause was a tip-off about the placement of a bomb.
Schroeder's friendship with Russian President Vladimir Putin and his defense of Russian policy in the Ukraine crisis have been criticized in Germany.
Schröder, who was German chancellor between 1998 and 2005, is now chairman of the supervisory board of Nord Stream, the gas pipeline connecting Russia and Europe across the Baltic Sea.
A previous celebration of Schroeder's birthday, sponsored by Nord Stream and attended by Putin, also caused controversy.
Schröder's photos, in which he is smiling and warmly greeting Putin, caused such a storm that the German government distanced itself from the former chancellor.
In an interview published by the Welt daily on Sunday, Schroeder said that the European Union is the main culprit in the Ukrainian crisis, as it forced Kyiv to choose between the EU and Russia.
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