When I try to install plugins via WPMU DEV Dashboard I get error 500.
During the chat we tried plugin conflict - no results.
I increase memory limit to 512M and max_execution_time to 100000 - no results.
I tried curl and wget from the command line to download a plugin. But they didn't work. When I look at the downloaded file, it looks like code for a login webpage.
--2017-12-29 07:57:46-- https://premium.wpmudev.org/download/372101913_custom-sidebars-pro-3.1.1.zip Resolving premium.wpmudev.org (premium.wpmudev.org)... 2400:cb00:2048:1::6810:180a, 2400:cb00:2048:1::6810:190a, 18.104.22.168, ... Connecting to premium.wpmudev.org (premium.wpmudev.org)|2400:cb00:2048:1::6810:180a|:443... connected. HTTP request sent, awaiting response... 302 Found Location: https://premium.wpmudev.org/wp-login.php [following] --2017-12-29 07:57:49-- https://premium.wpmudev.org/wp-login.php Reusing existing connection to [premium.wpmudev.org]:443. HTTP request sent, awaiting response... 200 OK Length: unspecified [text/html] Saving to: ‘372101913_custom-sidebars-pro-3.1.1.zip’ [ <=> ] 3,472 --.-K/s in 0s 2017-12-29 07:58:04 (36.3 MB/s) - ‘372101913_custom-sidebars-pro-3.1.1.zip’ saved 
[28-Dec-2017 18:40:21 UTC] WPMU DEV Upgrader error: INS.06 - Download failed. cURL error 18: transfer closed with 678304 bytes remaining to read.
I also try to install plugin from default Wordpress plugins section and this works correct.