Germany has begun publishing an online list of works that were discovered in a Munich flat last year.
The art is believed for the most part to have been stolen or extorted by the Nazis.
The move was welcomed by lawyers representing families whose looted art was feared lost forever.
But heavy demand for the government's "Lost Art" website led to technical problems that made it difficult to gain access.
"No one was expecting such a storm of demand," said a spokesman for the Culture Ministry.
"The server was overwhelmed by the massive demand. The only thing to do is wait."
A statement from the national and Bavarian regional governments said 25 of the works would be displayed initially on the "Lost Art" site, which helps to establish the provenance of works seized by Germany's Nazi regime, mostly from Jews persecuted during the Holocaust.
The government has been heavily criticised - notably by families whose relatives were robbed by the Nazis - for keeping silent for almost two years about the trove of 1,406 European art works until a German magazine broke the story last week.
Defending their policy of silence, government officials said they were worried about the security of the art works and the related insurance, and that authorities were also conducting a confidential tax fraud investigation into Cornelius Gurlitt, in whose Munich apartment the art was found.
Works by Picasso, Chagall and Otto Dix were among those on the government's website, according to German media.
In the United States, a retired lawyer who has a claim on one of the paintings - Two Riders on the Beach - by Max Liebermann, told Reuters that he hoped to be able to get back the work, which belonged to his great uncle.
"I want my painting back, and soon," David Toren, 88, one of the two heirs of David Friedmann, said.
Friedmann was an industrialist from Breslau who owned the painting from at least 1905 to 1939.
Mr Toren, who lives in New York, said he can still picture the painting that hung on the wall of his great uncle's villa before the war.
Friedmann died in 1942. Mr Toren escaped from Germany and spent the war years in Sweden. His older brother reached the Netherlands and now lives in London.
Their parents perished at Auschwitz.
The Liebermann painting was among the first art works posted online from the Gurlitt stash.
Berlin lawyer Lothar Fremy said he had been searching for the work for about five years on behalf of Friedmann's two surviving relatives.
Mr Fremy said he filed a claim with German authorities last week after the work was featured at the press conference held about the cache.
"Legally it's very complicated," he said. But he urged the authorities to work quickly, given that his clients, Mr Toren and his brother, are aged 88 and 92.
Legal status of art hoard likely to be contested
The hoard is estimated to be worth up to €1 billion and its legal status is likely to be contested.
Customs officials stumbled on it during a routine investigation in Munich's smart Schwabing district in February 2012.
"The origins of the so-called 'Schwabing art trove' will be traced as quickly and transparently as possible," the federal and state governments said yesterday - over a week after news of the find was reported by the Munich magazine Focus.
The paintings, sketches and sculptures hoarded by the war-era art dealer Hildebrand Gurlitt, put in charge of selling confiscated "degenerate" art by Hitler, were found in the apartment of his reclusive 79-year-old son, Cornelius.
But their legal status is ambiguous, nearly 70 years after the war in which the Nazis plundered hundreds of thousands of art works from museums and from individuals, most of them Jews.
The government's coordination centre for lost art said on the website that around 970 of the works were believed to have been confiscated, stolen or looted by the Nazis.
Some legal experts say Mr Gurlitt may get to keep the art, but others say Germany could nullify his ownership under the 1998 Washington Declaration, a set of principles for dealing with looted art.
The governments said they had set up a team of six experts to examine the provenance of the works.
The federal government, which ordinarily leaves such cases to regional justice officials, stepped up its involvement after the United States asked it to publish a list of the art works.