Answer:
In order to recognize a cross-section as an I-section in RF‑/STEEL EC3, it is necessary to define a cross-section of the I-section type. This is the case with rolled and welded I-sections from the cross-section library, but not for built-up or modified cross-sections. Therefore, an ICM section (a modified cross-section) can no longer be automatically recognized as an I-section, for example.
The easiest way to design a cross-section with the General Method is to change the settings. You can also activate the design for other cross-sections using the "Enable also for non I-sections" option.