draw.io is a very powerful drawing app, now called diagrams, currently included in fairapps+ (sandstorm) and in faircloud.eu (using the SaaS embed.diagrams.net service).
how to get simultaneous editing running?
- use google drive (works, needs to be started as a google drive extension, tested by @lisa.kostrzewa - every participant needs a google account - this is not an option)
- gitlab - not yet tested https://github.com/jgraph/drawio/issues/685#issuecomment-800278671 - is this in community version? to use git is not an option of non-tech users
- for nextcloud app a file sync mechanism has to be programmed https://github.com/pawelrojek/nextcloud-drawio/issues/74#issuecomment-859380095
- VScode extension https://github.com/hediet/vscode-drawio mit VScode liveshare extension (needs to sign in with Github or Microsoft account), supports up to 30 collaborators that also need to use Visual Studio Code and the two extensions. However in the VS web version this seems not to work so it takes too many steps for collaborators.
Ein einfacher file-lock mit auto-save und auto-load wäre vielleicht schon mal ein erster Schritt - in einer Videokonferenz kann man sich ja leicht ausmachen wer gerade zeichnen darf.