@fennsen some hours later the problem is solved:

indeed it was the combination of dewpoint devices and calculating expression variables;

The issue was caused by a combination of tricky circumstances:

The new dewpoint plugin seems to handle the non-discrete values in another way.
My calculations variables referenced to the xxx.humidity and xxx.temperature values of the dewpoint device.

As the migration on existing DB failed, I let pimatic setup a new database on startup.
As this are discrete values and in my setup only written every 5 minutes to DB, pimatic got confused during startup, because the values wasn’t some kind not right there at this time. So it seems that pimatic tries to calculate and waiting for the data (CPU raises to 100%… memory overflow). Depending on the sensors this needs some minutes to populate all the temp & humidity values; during this time the dewpoint values are not there and pimatic tried to calculate and wait in memory (endless loop i guess)

It took me days to get to this point where the real issue was.
I’ve had to fixed this by disabling all the relevant stuff upfront:

Opening a htop window in parallel to see exactly the behaviour and the concrete point at which the memory and cpu get crazy;

deleting all relevant devices, variables, and rules

Step by step enabling again:
2.1 add devices (->ok)
2.2 add variables 1by1 (->here it crashed the first time->root cause analysis)
2.3 add rules 1by1 (->here it crashed the second time->root cause analysis)

Long story short outcome - lessons learned:
Don’t expect a smooth migration to 0.9 if you have a very complex installation, a lot of rules and/or expression variables which might fail due to changed taxonomy and/or data types / other changes in the plugins.

disable IPV6 if possible (the listener sometimes switched the listening port to the ipv6 stack; client connected but fails to communicate) If you use MQTT: don’t use ‘localhost’ as the default. Use “127.0.0.1” or the dedicated IP-adress of the broker machine. If you have ipv6 stack enabled it’s a questions of luck which path he takes for connections

Good luck and have fun!