I have been following this forum post as I too am having the same errors when trying to create the remote repo via the GUI.
I have setup and tested ssh from client to server from command line all working ok. No password prompts etc all good.
I too can create remote repo as per instructions and all works ok. from command line only NOT via GUI.
I followed the guide as above in setting this up....
The thing that puzzled me was this instruction
export BORG_RSH='ssh -i /etc/ssh/my_secure_server_key'
i did try export BORG_RSH='ssh -i ~/.ssh/id_rsa'
this did not work either in the GUI
getting same errors as before
when I generated my key pairs it was placed in the users home folder in a sub folder .ssh with the default file names of
id_rsa <being private key>
id_rsa.pub <being public key>
I checked the /etc/ssh folder and there was no id_rsa or id_rsa.pub.
I did notice there were private and public keys from other systems placed there generated by OMV and plugins.
example
ssh_host_ecdsa_key
ssh_host_ecdsa_key.pub
etc..........
I then copied my id_rsa and id_rsa.pub to /etc/ssh folder
Renamed id_rsa to ssh_host_borg_key
and id_rsa.pub ssh_host_borg_key.pub
then chmod 600 on both these files so only the owner has access to these in my case borg user
export BORG_RSH='ssh -i /etc/ssh/ssh_host_borg_key'
re attempted to create again via GUI
ssh://borg@mountaineagle-technologies.com.au:7290/borg_volume/golfclub10
Getting same permission errors etc as before.
I then looked at the GUI and found a TAB that said 'environment variables '
nothing was listed
I then created the BORG_RSH variables in the GUI as below

note there is a drop down box on repo*
Repo creation
then will list any repo's you have created
select Repo creation
then save
Next create the remote repo
Select Repos the + to add

Click Save
Result

Now we can proceed to create Archive from GUI
But before we do we need to go back into environment variables TAB select BORG_RSH variable and change it from Repo creation to point to the repo you just created using the drop down box

If you do not change you will get authentication errors when you attempt to create archive as below

Point BORG_RSH to REPO as above eg golfclub_remote
Then select archive and execute with the right arrow

Success yes wallah !!!!!!!

So I think the environment variable BORG_RSH being set at the command line level isn't working somehow
as in instruction
export BORG_RSH='ssh -i /etc/ssh/my_secure_server_key'
I have being trying to nut this out for a long time. Reading guides and checking forums
Love the GUI to borg backup Thankyou to the writer
And now I can have a offsite repo's as well as local for security and redundancy
I don't know if the process I took is the correct way, but it all clicked together, tick, tick, tick