Nextcloud not reachable while uploading lots of photos

Nextcloud version (
Operating system and version (QNAP NAS using docker and Nextloudpi v1.28.1):

Every time somebody is uploading much data I am not able to reach the nextcloud till the upload is finished. Theres no way to connect to the website, via webDAV or over the app.
But there’s no CPU or RAM overload…

Could somebody help me?