Hi Michael,
thanks for your answer, althought it did not solve my problem ("rth"-attribute was already set and the order warning did not cause the problem). However, in case someone has the same problem in the future, Id like to share the solution:
In the TWS-configuration in the "presets" --> "stocks" section you find the box "Prefer SMART". TWS will SMART route all orders if Smart-routing is available, regardless of the routing selection defined for the ticker. And only if you navigate with the mouse over the small info-icon they tell you, that this also applies to orders send via the API!
I never changed this setting, so i dont know if the setting changed by itself in the last week or if the way the setting works changed. However: I checked the box and now everything works fine again.
Regards,
Daniel