Hello Jochen -
welcome to driver & firmware hell. This is very likely a problem related to stability issues with your WiFi interfaces. I would expect this behavior since you are using USB WiFi dongles. All USB WiFi devices depend on firmware, which is loaded when you initialize the devices. Ad-hoc mode has been neglected far and wide by the industry. Since you have to load a proprietary binary blob, which is more likely broken in ad-hoc mode rather than not, I am afraid you are out of luck. I'd love to get a USB WiFi solution in my hands that actually works in a reliable and stable manner. Unfortunately I know of none.
If anybody has something that actually works, I'd love to know.
Cheers, Elektra