Hi Dan and Natasha!
Thank’s again for the answer!
For me it was also necessary to update the Collation of the Columns to utf8_general_ci
ALTER TABLE `oauth_clients`
CHANGE COLUMN `id` `id` VARCHAR(254) CHARACTER SET 'utf8' COLLATE 'utf8_general_ci' NOT NULL ,
CHANGE COLUMN `secret` `secret` VARCHAR(254) CHARACTER SET 'utf8' COLLATE 'utf8_general_ci' NOT NULL ,
CHANGE COLUMN `name` `name` VARCHAR(254) CHARACTER SET 'utf8' COLLATE 'utf8_general_ci' NOT NULL ;
But it works for me.
Thank you!