I found https://community.nodebb.org/topic/10026/nodebb-plugin-write-api-failing-to-load-admin-page solving the issue.
SOLVED Max allowed file size setting not respected
Trying to upload 6M file returns: Maximum allowed file size is 4096 kB - please upload a smaller file
Nginx client_max_body_size 12M;
NodeBB Maximum File Size: 12288
You need to increase the limit from the NodeBB ACP as well.
Please correct me if i am wrong.. but i think i did..
unless Maximum File Size in settings > uploads relates to something else?!
Are you trying to upload a file in a post, or as a profile picture? The latter is a separate ACP setting.
Edit: I just tried again, and everything is working as expected. Thx anyway!
The file (mp3) is part of a post, not a profile picture.