Hmmm…
I think allow_url_fopen should be active/enabled in the php.ini – the plugin gallery tries to read an XML file from the remote URL https://pkp.sfu.ca/ojs/xml/plugins.xml and therefor fopen function is used that requires allow_url_fopen. So you could maybe check for that.
(Also, somehow strange that no error occurs, maybe you could somehow test if the error logging is working correctly…)
Best,
Bozana