Sorry for the delay.
Here are two most probable causes of the issue you're facing:
1. Your application passes "id_token_hint" url query parameter to `/end_session` endpoint - but this token cannot be found in Gluu Server's db/cache as session is no longer existing, and thus it can't be validated. According to the spec - found [here](https://openid.net/specs/openid-connect-rpinitiated-1_0.html) - "When an id_token_hint parameter is present, the OP MUST validate that it was the issuer of the ID Token", also "..If any of the validation procedures defined in this specification fail, any operations requiring the information that failed to correctly validate MUST be aborted". Thus, it turns out, in such case oxAuth behaves correctly, what was also confirmed by the dev team in the past on a similar issue. To mitigate against this, you may choose to not send the "id_token_hint" parameter in end session requests.
2. If you've confirmed you don't send the parameter, but include "postlogout_redirect_uri" parameter instead and still see that error, especially in case when you know that the session has already been ended by another app, it's probably due to the fact oxAuth can't find any session context for that request, thus it can't validate the uri as it doesn't have any idea what client's property it needs to check in such case. I can confirm that behavior and it indeed seems that error oxAuth displays is confusing. You can try to enable "allowPostLogoutRedirectWithoutValidation" option at "Configuration" > "JSON Configuration" > "oxAuth" page, and then add all possible "postlogout_redirect_uri" uri-s that may appear in such request to "clientWhiteList" list on the same page. This will make oxAuth to redirect such requests to the whitelisted uris without any extra validation.
Hope this helps.