Service unavailable only for nextcloud app

Servus :slightly_smiling_face:

we have the following problem:

If we use Nextcloud with a web browser or the Windows client, it works perfectly. But the problem is that we get Service unavailable (HTTP 503) with the current iphone release of Nextcloud in 80% of tried file transfers. I dont no why but sometimes it works.

Sorry but we have no log entries or something else.

Thanks for your help

Nextcloud version: 12.0.4
Operating system and version: Ubuntu 16.04.3 LTS
Apache or nginx version: Apache/2.4.18 (Ubuntu)
PHP version: PHP 7.0.22-0ubuntu0.16.04.1

If you have not solved this issue, what solved mine was downloading all the files from my account off the server onto my PC then deleting all the files on the server in my account then dragging the files I downloaded into my Nextcloud folder on my PC to resync to the server.