Internal error: Required parameter not passed: "code"

[torgeirl]

Login works but on redirect I get Notice: Undefined Index and Required parameter not passed: "code" when I try to use this extension. The OAuth2 provider I'm using (Dataporten; documentation) responds with the expected $response['user'][...] key so that shouldn't be a problem (issue #3).

In the start I thought it had something to do with PHP7 incompatibility, but I get the same result using PHP5.6, PHP7.0 and PHP7.1.

Setup
MediaWiki 1.30.0
PHP 7.1.12 (using php-fpm for Nginx)
PostgreSQL 9.5.10
ICU 55.1

Full errors (userid and state replaced with X's)
Notice: Undefined Index: code in /var/www/wiki/extensions/MW-OAuth2Client/SpecialOAuth2Client.php on line 98

/index.php/Spesial:OAuth2Client/callback?code=XXXXXXXX-XXXX-XXXX&state=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX BadMethodCallException from line 35 of /var/www/wiki/extensions/MW-OAuth2Client/vendors/oauth2-client/src/Tool/RequiredParameterTrait.php: Required parameter not passed: "code"

Backtrace:
#0 /var/www/wiki/extensions/MW-OAuth2Client/vendors/oauth2-client/src/Tool/RequiredParameterTrait.php(53): League\OAuth2\Client\Grant\AbstractGrant->checkRequiredParameter(string, array)
#1 /var/www/wiki/extensions/MW-OAuth2Client/vendors/oauth2-client/src/Grant/AbstractGrant.php(76): League\OAuth2\Client\Grant\AbstractGrant->checkRequiredParameters(array, array)
#2 /var/www/wiki/extensions/MW-OAuth2Client/vendors/oauth2-client/src/Provider/AbstractProvider.php(563): League\OAuth2\Client\Grant\AbstractGrant->prepareRequestParameters(array, array)
#3 /var/www/wiki/extensions/MW-OAuth2Client/SpecialOAuth2Client.php(98): League\OAuth2\Client\Provider\AbstractProvider->getAccessToken(League\OAuth2\Client\Grant\AuthorizationCode, array)
#4 /var/www/wiki/extensions/MW-OAuth2Client/SpecialOAuth2Client.php(65): SpecialOAuth2Client->_handleCallback()
#5 /var/www/wiki/includes/specialpage/SpecialPage.php(522): SpecialOAuth2Client->execute(string)
#6 /var/www/wiki/includes/specialpage/SpecialPageFactory.php(578): SpecialPage->run(string)
#7 /var/www/wiki/includes/MediaWiki.php(287): SpecialPageFactory::executePath(Title, RequestContext)
#8 /var/www/wiki/includes/MediaWiki.php(851): MediaWiki->performRequest()
#9 /var/www/wiki/includes/MediaWiki.php(523): MediaWiki->main()
#10 /var/www/wiki/index.php(43): MediaWiki->run()
#11 {main}