Lei Jie, chairman of China's Founder Securities Co.

Lei Jie, chairman of China's Founder Securities Co and its joint venture with Credit Suisse Group AG, could not be contacted by Founder as the government questions executives at the company's parent.

Lei asked for a week's sick leave on Jan 12 and the firm has not been able to reach him since Monday, Founder Securities told Shanghai's stock exchange on Thursday night. Lei could not be reached on his mobile on Thursday.

Founder Securities, China's sixth-largest listed brokerage, and parent Peking University Founder Group are embroiled in disputes with billionaire Miles Kwok's Beijing Zenith Holdings Co, the brokerage's second-largest shareholder. In a public trading of accusations since late last year, Founder denied Zenith claims ranging from insider trading to misappropriating State assets.

"This is a headache for Credit Suisse - it's a problem that nobody could have expected," Linus Yip, a strategist at First Shanghai Securities Ltd, said in Hong Kong. "One of the major risks for foreign firms in joint ventures in China is suddenly discovering some problem with the partner that is beyond their imagination."

Zenith obtained a court order freezing some Founder Securities bank accounts last month. This month, Founder Group executives were questioned by the authorities and its chairman stepped down, according to statements by its units.

The Swiss lender owns one-third of Credit Suisse Founder Securities Ltd, which was set up in 2008 and competes with similar partnerships between Chinese firms and global banks including UBS Group AG, Goldman Sachs Group Inc and JPMorgan Chase & Co. Foreign banks are allowed to underwrite stocks and bonds in China only through joint ventures with local firms.

The relationship between Founder Group and Zenith soured after a deal in 2013 that saw Founder Securities combine with China Minzu Securities Co, a private brokerage owned by Zenith.

Shares of Founder Securities, which is based in Changsha, Hunan province, have declined about 17 percent since Dec 8, when Zenith said it had made a report to the authorities accusing Founder Group management of wrongdoing. Even so, Founder Securities' shares have more than doubled in the past six months because of China mainland's stock market boom.

Global firms have struggled to make inroads into China's stock underwriting market, which was dominated last year by domestic players, with neither the Credit Suisse nor the Goldman Sachs joint ventures advising on any initial public offerings, according to Bloomberg data.

In 2014, the Credit Suisse joint venture ranked 17th among advisers for mainland equity, equity-linked and rights sales, the best ranking among global investment banks.

He Qicong was appointed acting chairman of Founder Securities this week and will remain in that job until Lei returns or the board sees a need to replace Lei, Founder Securities said in Thursday's statement, adding that it is in contact with regulators.