The problem is solved!
Positive result: I created an account on “plus.seafile.com” and everything worked perfectly—in both directions (up and download).
Somehow, I then looked at the settings on the iPhone, as I had left more or less everything in default on the server settings, except for the data directory and my local IP address.
However, everything is well documented. It “has to be” on the iPhone (iPad).
So I went through the iPhone’s settings once more. Step by Step.
I examined each setting individually in the iPhone’s settings. And this time I clicked on downloads and uploads.
Both displayed a “0,” (!) which always indicated to me that nothing was happening. It was foolish to assume that “nothing” was taking place.
When I selected “Downloads,” I discovered that some files were ones I had previously renamed (Update: the files were renamed on my Mac). Maybe during the download of the files to the iPhone, I don’t know.
Now I clicked “Cancel Download” and suddenly all the files were downloaded to my iPhone.
Tests in which I then uploaded images (via Seafile, from Apple Photos) were suddenly successful as well.
TL;DR:
- The downloads (and also the uploads) were blocked because they contained one or more files that were no longer available, but waiting for the download.
- After clicking “Cancel Download” everything works as expected and in the past.
- The same issue was on the other devices, because of the photos.
Yeah … I am happy again.
Is this maybe a bug?
EDIT: add. information (see Update)