I googled the error message and found multiple threads in the forums and one on GitHub, which include possible solutions:
https://github.com/nextcloud/desktop/issues/3707
TL;DR
Make sure the config.php
on your server contains the following lines:
'trusted_domains' =>
array (
0 => 'cloud.domain.tld',
),
'overwrite.cli.url' => 'https://cloud.domain.tld',
'overwritehost' => 'cloud.domian.tld',
'overwriteprotocol' => 'https',