Nextcloud ceases functioning as USB drive goes to "read only"

My nextcloudpi on raspberry pi 4 has been acting up lately. Nextcloud stops working and reason is that USB drive (8 tb seagate) goes to write only mode (data directory is on the USB drive). It can happen any time, so I just scrapped everything, started with freshly formatted drive and formatted memory card with clean install.
After install I created first user, started to sync (from local to nextcloud) and within minutes it had happened again. Client gives error on every single file, and going to server, the USB drive is in write only.
I’m out of ideas, please help.

Did you check your logfiles? Is there an extra power supply for the disk (direct or via USB hub)?