(Reuters) - Shares of Super Micro Computer ( SMCI ) tumbled more than 20% in premarket trading on Wednesday as an unclear timeline for its annual report along with weak quarterly forecasts fanned investor worries about the AI-server maker.

The company's auditor Ernst & Young unexpectedly stepped down last week after earlier highlighting some concerns over its financial reporting. Super Micro said on Tuesday an investigation by a special committee of its board did not find any evidence of fraud or misconduct.

"The actions of the prior auditor and the Special Committee are at odds to each other further increasing confusion around the current developments rather helping in regards to more transparency," J.P.Morgan analysts said in a note.

In late August, Super Micro also delayed the filing of its annual report, citing the need to assess "its internal controls over financial reporting".

That came a day after short-seller Hindenburg Research said it had taken a short position in the stock, alleging "accounting manipulation" at the company.

Super Micro risks being delisted from the Nasdaq if it does not meet deadlines later this month.

The company also forecast second-quarter sales and profit below Wall Street expectations on Tuesday, as it awaits the latest chips from Nvidia ( NVDA ) to be delivered.

Super Micro's shares have been choppy since peaking in March on a boom in generative artificial-intelligence technology, which has boosted demand for AI-powered servers and hardware used to process large amounts of data.

Its stock is down around 2% this year after rising more than 240% last year.

Super Micro trades at a forward price-to-earnings ratio of 7.56, compared with 14.70 for Dell Technologies and 9.51 for Hewlett Packard Enterprise ( HPE ).