i was in the EXACT same situation but on weblogic 10.3.3.
* two servers in FAILED_NOT_RESTARTABLE
* try to delete them from admin console and i get the "is a running server and my not be deleted..../incompatible with this operation" messages
i tried lots of things and but i could not get rid of those servers.
### What ended up working. ###
i just started the servers. there is no way they should have started. i had already even deleted the server directories. but they started. and then i shut them down. and at that point i was able to delete them.
hope this helps you or somebody. it seemed like such an unlikely solution. but it worked for me.
Edited by: user8825653 on Mar 15, 2013 8:25 AM
So let me get this straight...
You not only removed the instance (or tried to) from the admin console, but you deleted the directory structures associated to the instance.
Then eventually you went into the admin console, and told it to start the instance.
You were then able to stop it, and remove it from the admin console?
Hit this post when searching for this issue (FAILED_NOT_RESTARTABLE). In my case the issue was due to a port conflict with another managed server. I assigned an unused port number to my server and then I was able to start the server again ... I know the post is old but, perhaps it can help other people.