Please mention the exact error  message you are getting in ULS logs

Look into application logs for related error messages.

How to get the right set of ULS logs
===========================================

Run following command on the web  Front Sever serving the SharePoint site:

net stop sptrace
net start sptrace

These commands will force SharePoint to start logging to a new file.

Now try to reproduce the issue two-three times so that the detailed error
is captured in ULS logs.

This will actually force SharePoint logging service to start writing to a
new log file located in C:\Program Files\Common Files\Microsoft Shared\web
server extensions\12\LOGS

Sort on modified date a take a look at the latest log file to find the
error.