During posting the program requires exclusive access to certain files, but only for the moment. Although posting of an invoice would momentarily lock the files, they would be released immediately, and the other module could proceed. Given the message you receive, it sounds like some file is not released by OE and that is definitely a configuration issue. Have you checked data integrity? In a similar situation a bad network connection resulted in one workstation posting orders and locking out the others because it would send bad data to a file and then the file would be unusable.