Users Session Needs to be disconnected

dears,

i'm running a Widows server 2008 R2 Std. SP1 as a file server and host some applications based on MS access and VFP at the same time.

some users randomly complain they are not able to run the applications while thy are able to access the shared files at the same server and the time of the applications error.

disconnecting the users session to those applications makes the applications running correctly, 

any idea about this issue

Sofyan