Do you mind sharing which community it was? I will also play around with posts to see if i can reproduce it

I’m done investigating the issue. The cause appear to be when you are on a community but you are not using an account that is associated with the instance the community is on. This is a problem with the app not being specific enough.

Here’s an example. Let’s say you have two accounts: accountA and accountB. accountA is on lemmy.world while accountB is on lemmy.ml.

Let’s say accountA visits the summit community. accountA and summit are both on lemmy.world. There is no problem.

Now you switch to accountB. accountB is on lemmy.ml and summit is on lemmy.world. There is a problem because there is no summit on lemmy.ml. Instead you must specify the full name “!summit@lemmy.world”. So when you try to create a post on summit with accountB you will get an error that the community “summit” does not exist.

This is part of a bigger problem where the app is only sometimes using the full name of community and sometimes not. The complete solution is to have the app always refer to communities by their full name. Eg. using the previous example again, accountA and accountB can agree that “!summit@lemmy.world” exists.