Bielefeld, Germany, a modest city of 338,000 people, is in every way typical of a well-off German town, with an opera house, a castle and a tidy historic district.

In fact, the city is so impossibly bland, according to a well-worn gag, it can’t possibly be real; all evidence to the contrary must be part of a giant conspiracy.

“Have you ever been to Bielefeld?” the joke goes. “Can you name one thing about Bielefeld?” The answer, of course, is no.

Even Angela Merkel, the former chancellor, piled on, saying in 2012 that she had given a speech in the city, “if it even exists.”

Bielefeld has gamely played along, even working the teasing into its marketing. But it clearly irks local residents.

“Everyone laughs at us, because of this joke,” said Timo Teichler, the morning host on Radio Bielefeld. “Yeah, ‘Bielefeld doesn’t exist.’ I hate that joke.”

The Bielefeld Conspiracy began in 1993, around the dawn of the internet, as an experiment to see how quickly even an obviously ridiculous conspiracy theory could spread online.

The experiment became a proto-meme, and it made Bielefeld, in the dead center of the former West Germany, a byword for stuffy, hard-working monotony.

Bielefeld was the embodiment of old Germany when, in the decade after the fall of communism, many Germans were rushing to embrace a fast-changing Berlin and its “poor but sexy” cosmopolitanism.

Now, in a twist, at a time when the messy reality of 21st-century Germany is setting in, Bielefeld and all its boring stability, at least to those who live there, doesn’t look so bad after all.

The promises of unification has given way to a persistent, stark divide between east and west. The hope for enduring economic prosperity faltered in the face of competition from China. And Mrs. Merkel’s optimistic embrace of millions of refugees has fueled right-wing extremism and deep social divisions.

If Bielefeld is the German city that doesn’t exist, that is in part because it represents an ideal that many Germans feel may no longer be attainable.

It has an enviably diverse economy built on solid, family-owned companies. Its soccer team, Arminia, is even having a Cinderella season, helping to turn this whipping boy of a city into a national darling.

None of that means that even boring Bielefeld is immune to Germany’s social ills; this week a Syrian asylum seeker was arrested for allegedly stabbing and wounding five people celebrating the soccer team’s success outside a local bar.

Herbert Reul, the interior minister of North Rhine-Westphalia, the state where Bielefeld is situated, encouraged citizens to “be more vigilant” but he also told them “please don’t fall into a state of shock.”

Bielefeld, which prides itself on being welcoming, carried on.

“It is a modern and open city where diversity is lived and where diversity is even praised,” Pit Clausen, who has served as mayor for 16 years, said during an interview in his office, while his dog Scotty, a Labrador mix, slept under his desk.

Bielefeld is also the headquarters of the Bethel Foundations, a sprawling collection of hospitals and care centers for thousands of people with epilepsy, autism and other impairments. They live and work in the city and comprise a core part of the soccer team’s fan base.

To accommodate them, the city stadium has an abundance of wheelchair-accessible sections, as well as one of the world’s only viewing boxes for fans with autism, with volume controls and a calming room next door.

Arminia’s captain, a 30-year-old American midfielder named Mael Corboz, said the team drew inspiration from the city. The fans who packed the stadium, even when the team stumbled, helped the team overcome what for a time was its sad-sack status.

“When there’s a connection between the city and the club, there’s always something special that can happen,” Mr. Corboz said, standing on the sideline of Bielefeld’s 25,000-seat stadium.

Unlike most modern arenas, it sits amid a residential neighborhood. On game days, fans tailgate on their doorsteps.

“You can really say now that we have a team again,” said Heiko Lysek, a police officer who was shopping at the team’s retail store. “I’m 58 now. I’ve been waiting for something like this to happen for a long time.”

After Arminia beat the defending champions, Bayer Leverkusen, in the semifinals, residents unfurled blue-and-white bunting from houses and shop windows.

Outside the city, fans placed a giant jersey on the 87-foot-tall statue of the city’s namesake, Arminius, the German general who defeated the Romans at the Battle of the Teutoburg Forest in 9 A.D.

“We’re usually dismissed as a small town,” Mr. Teichler said. “Now everyone can see how big and loud and happy we are, and this is the first time we can feel such immense pride.”