If I can't delete a contact
An Olvid contact who changes device without restoring his backup necessarily creates a new Olvid identity (ID) on the new device.
Unless he kept the first device and uses it, his first ID is useless to anyone. It is outdated and he won’t receive anything anymore. This explains why creating the channel with the old ID of this contact will never end if you also restore a backup.
All contacts of this contact with an expired ID should also delete it from their contacts and add it with its new ID.
You can only delete a contact if you are not part of a group in common
An administrator of the concerned group must remove this contact (and his outdated ID) and invite his new ID.
An administrator of a group can delete members/participants
Tap (second last icon at the bottom) to display the list of Groups. The ones you have created and for which you are an administrator are displayed first.
Tap the name of the group. The details of the group and the members/participants are displayed.
Tap Remove Members.
Select the members you want to delete.
Validate with OK.
A group administrator can invite the contact and his new ID
When entering the group, the contact (with the new ID) will be automatically added to the contacts of the other participants/members of the group.
An administrator should advise the other participants/members of the group, via a message in the group, to remove this contact with the outdated ID from their contacts.
In the special case where the only administrator in a group has himself lost his ID, a new group needs to be created.