File Sharing and Access Problems only from ONE iMac

Hello,

we have several apple computer with OS X 10.7.5 in our Family:

1 iMac (mid 2007)

1 iMac (late 2011)

and 2 Mac Books...

We use alle the Computers for working with the same Email-IMAP-Accounts, same iCloud Calendars and same iCloud Address Book with only one user account because we don't want to tie a person to a specific device or change the user all the time. And we intensively use a shared folder on the old iMac, which is accessed from all computers. Because of the slow speed of the old iMac we decided to upgrade the newer iMac late 2011 with an 256 GB SSD and use it for file sharing in the future. An Apple Authorized Service Provider installed an Crucial M4 256 GB SSD and we got the iMac back with a blank OS X 10.7.4 allready installed. After a lot of Software Updates i used a Time Machine BackUp from the old iMac to transfer all the files and settings from the old iMac to the new ugraded iMac. Then i changed the device name of the old iMac in the sharing settings and disabled file sharing and printer sharing just to be sure that no one is accessing the shared folder of the old imac anymore.

My problem is that the old iMac has write access problems in the shared folder on the recently upgraded iMac. The account which shares the public folder is the administrator account we used allready on the old imac(it was restored by the migration wizard) and i use exactly this username and password on all computers to connect to the shared folder from the upgraded iMac. Before changing the computer used for sharing everything was fine. After using the upgraded iMac for sharing only the old iMac seems to have write access problems. If i open an excel sheet, only on the old iMac this excel sheet is marked as read only and there is no way to unlock it. When i try to delete a file from the shared folder through the old imac, i first have to type in the name and password of the administrator account to delete the file.

On the upgraded iMac i tried to apply read/write permissions of the public folder to enclosed items in finder for all 3 entries(admin, staff and everyone). Then i checked the sharing configuration and all 3 entries(admin, staff and everyone) have allready read/write permissions. I also tried to repair Disk Permissions in the Disk Utility, but the problem remains unsolved.

So what could be the cause that only the old iMac seems to have write access problems, allthough i use the same username and password for the network connection on all computers?

What can i do to fix this problem?

Thanks in advance,

CardiacInfarction