Workaround found here https://xcp-ng.org/forum/post/94626 Seems like turning "Use NDB to transfer disk if available" switch ON and save backup job, then turn the switch OFF and save backup job again, will fix the issue. Thanks to @RobWhalley and have a nice day everyone.

@gduperrey said in XCP-ng Tools Storage contains VDIs I’m unable to Destroy: It seems that it is possible to clean old isos manually, via the command line, as explained in this forum post: https://xcp-ng.org/forum/post/15703 Wau!!! That actually works like charm! Thank you sooooo much!