Service Unavailable error on Theme 0ptions

Hello,
We have been using the Blackfyre theme for quite some time, and never had any issue until recently. Since a few weeks ago (can’t pinpoint the exact date), we get an error whenever we try to edit the Theme options.

Symptoms:
– When clicking ‘Save all changes’ in Theme Options, we get the following error:
“Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.”

– This error occurs regardless of which changes are we trying to save (eg. the logo, the ticker items, or even with no change at all). The attempted changes aren’t saved.

– We enabled WP_DEBUG, and in certain other admin pages (eg. Users) we are now seeing the following error message at the top of the WordPress dashboard, which may be related:
“Notice: Undefined index: login in […]/wp-content/themes/blackfyre/functions.php on line 4034”

– In spite of seeing that error, all other WordPress features are working correctly. The Theme Options are the only thing we cannot edit right now.

Other information:
– We have never used Clans or let anyone register. There are only two Users, both admins. We’ve tried logging out and logging in. There were two other admin users that have now been deleted.

– We are using WordPress v5.4, although the problem started happening on v5.3. Updating to 5.4 didn’t make any difference.

– We have updated to the latest version of the theme available on Themeforest, as well as Blackfyre types (1.6), BuddyPress (5.1.2), and all other plugins used. We have never installed most of the other recommended plugins (BBPress, Disqus, WooCommerce…)

– We didn’t have the BuddyPress plugin installed when the problem started happening. We’ve only added it later to check if it would fix the issue.

– We are using several other plugins, all of which are up to date. We have tried disabling all of them, and the ‘Service Unavailable’ error kept happening. Let me know if a list of them would help.

– The WordPress installation is hosted by Ionos.

Please let me know if any other information would help, or if there’s anything we can do to get around this situation!

Thank you very much in advance!