@pitaj yes, I tried ./nodebb build several times, but I always used ./nodebb dev to start.
I tried it once more as you said. ./nodebb start aborted with the message to run ./nodebb upgrade. After the upgrade and another ./nodebb start I can post and answer again! 😄
I don't know why an how, but it works again. hopefully not only for a short time. 😉
Thanks for you help!