File Access slow

Nextcloud: 13.0.4
PHP 7.2.1
MySQL: 5.6.38

For some weeks now I am having problems accessing my files on two different instances of Nextcloud on two different servers:

I call the Interface shows up, but the file list keeps on loading and loading.

I click on the house icon the URL changes to and the files show up immediately.

I click one of the folders: loading and loading

I click on the folder name in the breadcrumbs: zapp, the files are there. (often - not always)

I disabled all apps: No changes

Same behaviour on both Nextclouds…