Error detected at code line #5717 of method 'awajax.formrestoreuser'
Method or procedure userpropstablesdelete(): error #1705 ("File access is denied c:\windows\temp\_47y0k7jn7_main_scx_old.cdx.") at code line #4255
main_scx

URL: http://frontend.acut.it/acuterp/MethExec.erp?&MethAddr=InitControlsInet&ObjAddr=main_scx&PageInit=true&UserID=&nReq=1
Client: 188.10.142.44
Form variables: &lastKey=0&version=2.1&sync=true&vpw=1167&vph=565
Call stack: 1.�processhit > 2.�process > 3.�routerequest > 4.�erpprocess.process > 5.�wwprocess.process > 6.�erpprocess.methexec > 7.�erpprocess.ajaxformrequest > 8.�awajax.formrequest > 9.�awajax.formrestoreuser

Which problem can i have in my app when this error occurs ? The user doesn't see anything...but sometime i have the problem that a user can see data of other logged user, like some personal property (saved in forms.wcPropSave property) are not correctly saved....This is a situation i can absolutely have....It is better an error and a block....

This error occurs when you run multiple COM server and user hits the same page twice: both COM servers attempt to use the same table exclusively.
If your site has slow response times, user looses patience and hits the same URL twice or hits F5, hence the problem.

In the above case, User gets an error and there is no risk user data are confused.
Seeing data of other logged user never happens on other FiC sites in production (using up to 6 COM servers)
There might be something specific to your app
eg if you've defined the personal property in your form class .wcPropSave property, are you sure that all derived forms properly inherit from this property? for example some form might implement .wcPropSave differently and not inherit the personal property defined in the class.