FRANKFURT (Reuters) - A rare public boardroom dispute in Germany escalated on Friday, with the outgoing CEO of a major online broker criticising a company founder who had publicly campaigned against him.

Frank Niehage, the CEO of broker FlatexDegiro, this week suddenly announced he would leave after 10 years at the helm of the firm that operates in 16 countries with 2.7 million customers.

The departure followed sharp criticism from the company's founder and major shareholder, Bernd Foertsch, who told a business magazine last month he would vote against Niehage and the company's chair at an upcoming shareholder meeting due to strategic mistakes, like "sleeping through a cryptocurrency boom".

It is rare in the staid corporate culture of Europe's largest economy for such disagreements to occur in public rather than behind closed doors.

Niehage told a regularly scheduled earnings call with analysts on Friday he had resigned to "avoid further damage" to the company's reputation and to "resolve tension" with Foertsch.

He accused Foertsch of trying to get a seat on the supervisory board "through the back door, claiming to know better what's good for the firm", and called on shareholders to vote in favour of the chair at their meeting.

"Please register yourself for the upcoming annual meeting and exercise your voting power," Niehage said. "It is like in democracies: if you don't vote, you support minorities."

Niehage, Foertsch and the chair didn't immediately respond to emails seeking comment. A FlatexDegiro spokesperson didn't respond to repeated requests for comment.

The broker, which was fined by its regulator in 2023 and told to fix "serious shortcomings" in its internal controls, has steadily increased its profits over the past year. On Thursday, it posted a 340% increase in first quarter net profit from a year earlier.

Foertsch, in last month's interview with WirtschaftsWoche, criticised FlatexDegiro for not offering cryptocurrencies like Bitcoin directly through its platform.

Niehage had in the past expressed scepticism about such offerings, saying in October: "We are still in a crypto winter".

Stephan Simmang, one of FlatexDegiro's two interim CEOs, told journalists on Friday that Niehage "had an opinion on crypto that many of us didn't agree with".

By Tom Sims