* Fortum owns 49.99%, can't buy more
* Uniper, Fortum CEOs hold regular talks
* Knight Vinke has threatened to call EGM next month (Recasts on Fortum relationship, adds context)
By Christoph Steitz and Tom Käckenhoff
FRANKFURT/DUESSELDORF, Germany, Aug 8 (Reuters) - German energy firm Uniper is holding constructive but complex talks with top shareholder Fortum over their future cooperation, its new chief executive said, signalling slow progress in attempts to end a long-standing deadlock.
Finland's Fortum owns 49.99% in Uniper and has been trying to acquire a majority against the German firm's will. Russian regulators have blocked this, arguing a water license held by Uniper's Russian unit cannot be majority-owned by Fortum.
Uniper boss Andreas Schierenbeck, in the job since June, has been tasked with working out a solution for the situation, which also involves activist funds Elliott and Knight Vinke, jointly holding a stake of nearly 23%.
"We're optimistic about the future development as both sides share the interest and finally see some progress," Schierenbeck said about Fortum on Thursday after presenting first-half results that showed operating profit nearly halved.
Schierenbeck said he had spoken regularly with Fortum CEO Pekka Lundmark following a presentation about a month ago, when Lundmark outlined his strategy and plans to Uniper's supervisory board.
He said the two companies had already looked at their combined portfolio, adding Fortum has a stronger position in solar, while Uniper has a better presence in gas.
Schierenbeck signalled less flexibility with regard to the water license of subsidiary Unipro, effectively a poison pill preventing Fortum from buying all of Uniper, saying Fortum had been fully aware of the problem before making a bid.
"Moreover, Uniper – before Fortum formally presented its takeover offer – actively alerted Fortum to this legal restriction in Russia," Schierenbeck said. "To present this legal hurdle months later as a new problem neither makes sense to me nor does it fit the facts."
Fortum's Lundmark is also in talks with Russian authorities to get the hurdle removed.
To force through progress, Knight Vinke in June said it would seek to convene an extraordinary general meeting in September unless the two parties make significant progress in the coming months.
The fund said an EGM should give investors the opportunity to decide on a spin-off of the Russian business activities, which would remove the hurdle for Fortum to buy the rest of Uniper.
Uniper's Chief Financial Officer Sascha Bibert, in a call with journalists, said he had so far not received a motion to convene an EGM, invitations for which would have to be sent out six weeks in advance. ($1 = 0.8922 euros) (Editing by Michelle Martin)