I needed to increase the buffer of an open project ‘Window > Audio/MIDI setup’ but it triggered a crash so in case this happens to others:
I made a new project without any data and changed its buffer size, then I opened the problematic project from the ‘Session > Open’ menu of the new project and the project I wanted to change the buffer for opened with the new buffer size without crashing.
Something else that might have worked would be to turn off the Autostart in the ‘Window > Audio/MIDI setup’ , saved the problematic project and be offered the buffer settings options upon restarting Ardour and the project. I might keep Autostart in the off position from now on.