SUMMARY
Unable to complete oauth process with an account at https://pixelfed.social as indicated at https://kde.org/for/activists/.

STEPS TO REPRODUCE
1. Attempt to add account with pixelfed instance url: https://pixelfed.social
2. Click the authorization link  
3. Click Authorize in browser
4. Select Tokodon when asked to select an application to open "urn link"

OBSERVED RESULT
Receive error message in Tokodon: 

Failed to contact server: Error transferring https://mastodon.social/api/v2/search?q=urn:ietf:wg:oauth:2.0:oob?code%[...truncated...]&resolve=true&limit=1 - server replied: Unprocessable Entity. Please check your settings.

EXPECTED RESULT
Pixelfed account is added successfully

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: EndeavourOS
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.106.0
Qt Version: 5.15.9

ADDITIONAL INFORMATION
Using Tokodon Flatpak