[Schulisco]

I would give it a cross check with brake cleaner or start spray: Just one shot into the airbox (removed air filter) before start after sitting a night. If it right starts away as usual there might be a problem either on the fuel supply or the csv /tts side.
Did measured how long it takes the fuel pump to repressurize the fuel system while cranking?
On a hanging check valve or a bad / hanging fuel accumulator you probably experience the same behaviour. I don't think that the WUR is a problem while engine runs fine after a successful start. Nevertheless proper fuel pressure in spec is key.

Thomas