Cant assign user to card?

Nextcloud 20.0,0
PHP 7.3
Deck 1.1.2

When I go to a card and try to set a user in my NextCloud install nothing happens. It adds the round icon of the user at first but if I close the card details and enter the card again, assignment are lost. Am I missing something fundamental here? The Tags input field above works fine there with the colored "Action needed", "Finsihed" etc.

Ive tried Firefox, Chrome and Edge browser. Same behavior in all. I just cant get assignment to work at all for any card.

My install is a Unraid server with the NextCloud docker.

Error log:

[PHP] Error: Error: Undefined offset: 3 at /config/www/nextcloud/lib/private/legacy/OC_Helper.php#548 at <<closure>>

 0. /config/www/nextcloud/lib/private/legacy/OC_Helper.php line 548
    OC\Log\ErrorHandler::onError(8, "Undefined offset: 3", "/config/www/nex ... p", 548, {path: "/",rootI ... }})
 1. /config/www/nextcloud/apps/settings/lib/Settings/Personal/PersonalInfo.php line 108
    OC_Helper::getStorageInfo("/")
 2. /config/www/nextcloud/apps/settings/lib/Controller/CommonSettingsTrait.php line 141
    OCA\Settings\Settings\Personal\PersonalInfo->getForm()
 3. /config/www/nextcloud/apps/settings/lib/Controller/PersonalSettingsController.php line 77
    OCA\Settings\Controller\PersonalSettingsController->formatSettings({10: [OCA\Settin ... ]})
 4. /config/www/nextcloud/apps/settings/lib/Controller/CommonSettingsTrait.php line 152
    OCA\Settings\Controller\PersonalSettingsController->getSettings("personal-info")
 5. /config/www/nextcloud/apps/settings/lib/Controller/PersonalSettingsController.php line 68
    OCA\Settings\Controller\PersonalSettingsController->getIndexResponse("personal", "personal-info")
 6. /config/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 169
    OCA\Settings\Controller\PersonalSettingsController->index("personal-info")
 7. /config/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 100
    OC\AppFramework\Http\Dispatcher->executeController(OCA\Settings\Con ... {}, "index")
 8. /config/www/nextcloud/lib/private/AppFramework/App.php line 152
    OC\AppFramework\Http\Dispatcher->dispatch(OCA\Settings\Con ... {}, "index")
 9. /config/www/nextcloud/lib/private/Route/Router.php line 308
    OC\AppFramework\App::main("OCA\\Settings\\ ... r", "index", OC\AppFramework\ ... {}, {section: "perso ... "})
10. /config/www/nextcloud/lib/base.php line 1009
    OC\Route\Router->match("/settings/user")
11. /config/www/nextcloud/index.php line 37
    OC::handleRequest()

GET /settings/user
from 188.126.94.70 by claes at 2020-10-23T12:59:24+00:00

Reproduce card-user assignment in Deck 1.1.2 with the listed Nextcloud and PHP versions, then inspect the browser and server logs around saving and reopening a card. The related server trace names lib/private/legacy/OC_Helper.php and apps/settings/lib/Settings/Personal/PersonalInfo.php, but its connection to Deck should be verified. Done means an assigned user remains visible after closing and reopening the card.