File upload error to server (not available) with iOS (allways newest)

I love the Nextcloud System, and EVERYTHING works fine (ios, Mac, Windows, Android)
But SOMETIME a error from Nextcloud 13.xx to 17.xx still remains: file upload - server not available (translated from german) I dont know the exact English error message! Everything works, but the error most time occured when the ios app started, and try to upload a couple of pictures.

I think it’s a timeout, maybe the hdd has to wake up from standby, and nextcloud don’t wait for it.
Any ideas ? Machine: Raspberry 4B 1GB, but in a few days with 4GB and WD 2TB 2,5" inch HDD. I have a 256 GB SSD left for important actual needed files. But there is no priority system feature, i think. (like Mac SSD/HDD). But maybe it’s easy to transfer files between FAST and SLOW HDD/SSD ? I don’t try more than one drive.