What is causing "Cannot fast-forward to multiple branches" during `git pull`?

Question

We use the default workflow on GitHub, i.e., create a feature branch, push some commits to it, merge to master. Nothing fancy.

In the master branch, I only ever pull newer changes from GitHub, never push. In visual terms, the "down" direction in VSCode typically contains some commits while the "up" direction doesn't:

Now, from time to time (not always!), invoking the sync in VSCode which is git pull origin master ends up with this error:

> git pull origin master
From https://github.com/shoptet/sofa
 * branch                master     -> FETCH_HEAD
fatal: Cannot fast-forward to multiple branches.

What does the error mean? I don't see any multiple branches.

It's important to note that after I run the operation again, it works. So my workflow usually is:

1. Click the "sync" button in VSCode (runs git pull origin master)
2. See the "Cannot fast-forward to multiple branches" error, dismiss it.
3. Click the "sync" button again, see it succeed.

A bit annoying.

BTW my repo is in a clean state:

$ git status
On branch master
Your branch is behind 'origin/master' by 2 commits, and can be fast-forwarded.
  (use "git pull" to update your local branch)

nothing to commit, working tree clean

One additional thing that can be related as I think this problem only started happening after I enabled it is ff=only in my Git config – the full section looks like this:

[pull]
    rebase = false
    ff = only

I also looked in the VSCode logs and there's an additional git fetch before – I'm not sure if this changes something or not:

[2022-05-04T06:27:11.853Z] > git fetch [2373ms]
[2022-05-04T06:27:11.853Z] From https://github.com/shoptet/sofa
 * [new branch]          1997-fix-pnpm -> origin/1997-fix-pnpm
 * [new branch]          debug-build   -> origin/debug-build
[2022-05-04T06:27:11.892Z] > git symbolic-ref --short HEAD [3ms]
[2022-05-04T06:27:11.897Z] > git for-each-ref --format=%(refname)%00%(upstream:short)%00%(objectname)%00%(upstream:track) refs/heads/master refs/remotes/master [4ms]
[2022-05-04T06:27:11.901Z] > git remote --verbose [3ms]
[2022-05-04T06:27:11.905Z] > git for-each-ref --sort -committerdate --format %(refname) %(objectname) %(*objectname) [7ms]
[2022-05-04T06:27:11.909Z] > git config --get commit.template [3ms]
[2022-05-04T06:27:12.498Z] > git pull origin master [1918ms]
[2022-05-04T06:27:12.498Z] From https://github.com/shoptet/sofa
 * branch                master     -> FETCH_HEAD
fatal: Cannot fast-forward to multiple branches.

I'd like to understand what the error means and how to get rid of it.

I'm on Git 2.36.0 and VSCode 1.66.2.