Here are the promised first hand information based on my experiences made.

Bevore arriving to Baku in Azerbeidjan on Friday July 21, l got in contact with a very nice local person named Ismayl (mobile/whatsup +994 552861200) who operates his own travel agency in the city. He has excellent contacts to all the Caspian Sea Ferry officials and can give you first hand information about all the ships arriving and leaving the Azerbeidjan shores. If you contact him, please give my kind regards from the two crazy Swiss motorcyclist. We spent three unforgettable days in Baku together, due to him! So much fun in the wonderful city of Baku we did not expect at all!
First of all please note that there is no timetable available for any of the Caspian Sea Ferries since all ships are cargo ships and therefore are not specifically designated to transport tourists. They basically leave when they have enough freight at their disposal and that depends on the number of trucks or railroad carriages waiting at the harbour - either in Baku or at the "new port" at Alat ca. 60km south of Baku.
Usually tourists go to one of the harbours and ask when the next ferry will leave to either Turkmenbashi in Turkmenistan or like in our case for Aqtau in Kasachstan. This can go on for days. We have seen people staying in their tents at the harbour for days. Not sexy at all, believe me this is why I mentioned the port(s) in brackets. There is absolutely nothing than nothing to get or buy. So we choose the help of Ismayl sice he could us explain in detail the whole procedures on where and how to get the tickets on site. Remenber: Ismayl does not sell you tickets at all and he also does not own ships yet ;-))
So we arrived on a Friday an we could have left straight away the same afternon to Aqtau. Since we wanted to see Baku we've been told the next freighter would leave on Sunday. So fare so good. But on Sunday they said it will leave on Monday. So we spent the whole Monday waiting for the ship to get into the harbour, since the pot was on open waters yet and not allowed to come in due to very heavy and strong wind, which obviously happens quite often here. Most unpleasant for those on board who wanted to leave the ship finally...
So we got all the tickets and did all necessary procedures EXEPT CUSTOMS!
Once you have made customs you are no longer allowed to leave the harbour area. In our case we got back to our hotel an came again the next day - Tuesday now... We finally got on board by midnight and were lucky to get a TwoBedCabin with a window and not one of those inner cabins, know as Vietnameese Sweat Boxes like the ones the vietkongs used for torturing their enemies...
The ship left the next morning and fortunately the sea was absolutely calm. The passage took us 43 hours(!) in total since the pot runs with only 11 knots... It takes ages....
For two loaded motorbikes (BMW R1200 GSA), two passengers in a TwoBedCabin plus port taxes we paid US Dollars $ 390.- in cash. Others then cash payments are not accepted! The price includes fullboard - breakfast, lunch and dinner. The food on board was quite good, this in contradiction to what we expected an read on Internet comments by other travelers.
We learned that there is only one ship who runs between Alat and Aqtau. It's name is "MERCURY 1". The ship is as ugly as your phantasies can carry your imagination. Everything is rotten... If you start thinking about safety you've lost the game - about hygienic you're dead... so don't!

But the best of all was the Kasachstan customs control:
After entered into the port of Aqtau military came on board, fully armed together with a tracker dog sniffing for drugs. The started checking everything allready on board. Then we were allowed to leave the ship - frist without the bikes or cars! - and to do customs. Then back on board again to get the vehicles. Then again customs inspections of the vehicles. After that it started with endless personal customs procedures with countless papers to fill out, to pay for etc. etc. My traveling partner was quite close to a heart attach and allmost applying for a oxygen tent with heart massages...
Up on a certain stage I stopped counting the checks and controlls and I ended up in a psychedelic stage where you do all they tell you to do, like a telecontrolled robot. It's a kind of survival stage which reminds me on my years in the army... I never ever imagined those experiences might be any helpful to me once again in life - up to Central Asian Customs ....
The whole clearance took us 6 hours (!) and we've been told that this was quite fast. An Italian couple spent the whole weekend on customs.... We finally left by midnight and had to drive to downtown by night. When you know the Kasachian style of driving vehicles, quite an other adventure.

We did this journey since Turkmenistan refused our application for a transit visa. Otherwise we would have traveled via Iran and Turkmenistan to Uzbekistan. Most travelers we've met didn't get visas from Turkmenistan this year exept all those participating the Mongolian Ralley - strange enough.
Anyway - it's an experience by its own and feel free to do it - you will remember it for the rest of your life, that's for sure ;-))
If I would do it again? Yes - no doubts. But not tomorrow - please give me some time since all the negatives on a journey tend to fades away over the years and you begin to glorify all the issues experienced.

Cheers Silvio