I have the same problem on about 10 PC.
Our network is on samba domain, but some PC are not on domain, they are in workgroup.
Lansweeper work fine on domain, the server turn with domain user rights as "domain\user"
I had to add on all PC this user to administrator group.
The local user on different PC with the same name are not equivalent, because the hostname is different.
So hostname1\admin and hostname2\admin it's not the same user.
In the Premium version you can alternate the credentials, so your server can connect to PCs as hostname1\admin or hostname2\admin
But it's not work in my case.
May be you will have more luck.
Lansweeper don't support the domain or workgroup, so it's useless to wait for help from the support
ps. Sorry for my English 😄