I recently installed NC 12 on my Raspi 2.
Everything is working fine, so I decided to move the data-directory from /var/www/nextcloud/data to a mounted share on my NAS: /mnt/nextcloud.
After changing the config.php, I get the above mentioned error when I login to Nextcloud: net::ERR_CONTENT_LENGTH_MISMATCH when the server.css is being loaded.
The result is a missing CSS and a very poor display of the nextcloud frontend.
When I configure the data directory back to its original place, the stylesheet is loading fine.
This somehow happened to me during upgrade. It seemed that gzipped file server.css.gzip was corrupted and Chrome had some problem with that (other browsers were okay).
I fixed this by manually gzipping server.css and replacing server.css.gzip.
I have a similar problem with an instance migrated to NC15:
GET https://oc.tamm-tamm.de/index.php/css/core/0330-14c0-css-variables.css?v=0a45c4b720591db8aef57e8971e7b6f8-3f6b59b3-16 net::ERR_CONTENT_LENGTH_MISMATCH 200 (OK)
also leading to CSS elements missing and an almost unusable user interface. However, only Chrome is affected. Creating a server.css.gz file did not solve the problem. Any ideas?