Thank you jup and shoebox! I wrote to Metz and they very quickly replied the following which confirms what I thought was at the origin of the issue and states that the compatible firmware will be released this fall:
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We regret the trouble that you have with your flash unit "mecablitz 50 AF-1 P digital" and your new digital camera "Pentax K-30".
Sometime ago the new digital camera "Pentax K-30" was tested in our developing department. Unfortunately, we noticed that flash unit's current firmware version 3.0 is not suitable for running with the new digital camera "Pentax K-30". For this reason there are also not any flash unit recommendations mentioned for the "Pentax K-30" on our homepage.
The current firmware version of flash unit "mecablitz 50 AF-1 P digital" needs to be improved for the digital camera "Pentax K-30". A new firmware version for the flash unit is in preparation.
According to our developing department it is planned for the fall to release the new firmware version for the flash unit "mecablitz 50 AF-1 P digital" for running also with the new digital camera "Pentax K-30". Unfortunately, at the moment we can't supply you an exact date when the new firmware version will be available.
As soon as the new firmware version for the flash unit is available it will also be ready for download on our homepage:
Pentax: Metz
Additionally, we will also inform you by e-mail when the new firmware version for your flash unit is available.
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I also saw this
Field Upgradeable via USB - Pentax P-TTL Flash Comparison which tells me that the guys at Metz have to reverse-engineer the P-TTL digital protocol for each camera. So I guess for the next few month I'll have to use the manual mode or revert to my "old" K-x.