luceos Sorry, I'm not really sure how to interpret this. I've copied and pasted the entire response. I changed the url and server IP address. If there's any sensitive information in it please let me know or kindly remove it yourself.
I'm guessing it's an issue with the server, nginx? I checked the access log and it does not mention any errors.
As far as I know, I don't have a config.php file. The Flarum instance is new.
General
Request URL: https://forums.mysite.com/api/mybb-to-flarum
Request Method: POST
Status Code: 500
Remote Address: [myserver]:443
Referrer Policy: no-referrer-when-downgrade
Response headers
content-type: text/html; charset=UTF-8
date: Sun, 14 Mar 2021 02:27:39 GMT
server: nginx/1.14.0 (Ubuntu)
strict-transport-security: max-age=15768000
Request headers
:authority: forums.mysite.com
:method: POST
:path: /api/mybb-to-flarum
:scheme: https
accept: */*
accept-encoding: gzip, deflate, br
accept-language: en-US,en;q=0.9
cache-control: no-cache
content-length: 127
content-type: application/json; charset=UTF-8
cookie: _csrf=gkX_4Cq386rD-wPXWOSRt0VF; _ga=GA1.2.2023641700.1612589209; __profilin=p%3Dt%2Ca%3Dc4c2c3c0a8d3ba502a8731b43d474951; _forum_session=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--a0664c979af0b408037c71ca5fd79e25cc45c456; _t=2cd0a31f0d551065cfdc251870b05627; io=8zbz4F-BDXy7RBYsAAAE; flarum_remember=sLGk1fPKmoCywV8b2BQaW9ZO6h6eYgiXaoEyo5D1; flarum_session=PMvFdhAUZTPm50bsqzbKAFDXIjgIXt12NWHnoPem
dnt: 1
origin: https://forums.mysite.com
pragma: no-cache
referer: https://forums.mysite.com/admin
sec-fetch-dest: empty
sec-fetch-mode: cors
sec-fetch-site: same-origin
sec-gpc: 1
user-agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/IP_ADDRESS_ Safari/537.36
x-csrf-token: ZYmuqDclwdjUagaWzNwdXpZX3CrX4EsvtU2wT1jw
Request payload
{avatars: true, softposts: false, softthreads: false, doUsers: true, doThreadsPosts: true,…}
avatars: true
doCategories: true
doGroups: true
doThreadsPosts: true
doUsers: true
softposts: false
softthreads: false