LONDON—When his first tech venture fizzled more than a decade ago, Dan Wagner became Britain’s poster boy for the early 2000s dot-com bust—best remembered for a Donald Duck vest he wore to a news conference ahead of his startup’s initial public offering.

Earlier this year, another of his companies blew up spectacularly amid accusations of overspending on lavish offices and partying. Mr. Wagner, for the second time, became London’s face of startup folly.

Now...