As far as I understand the error message saying "Error loading USE or USELSX module" occurs in one typical scenario which is as follows.
You define a function. Put that in a script library and call the same on a event, say a button click on a form.
Later for some reason say you have been updating the function in the script library pretty often.
So the invoking code would be of a comparitively older version when compared with the one in the script library. This is one scenario where this error occurs...
There are other scenarios where this would occur. For example delare a constant in the form globals, say
CONST MYCONST="my value"
Now create a script library. Put the above mentioned code in the declarations section of the same, save the same and include that on the form which we created a before.
Now since the same constant has been defined in multiple places, you will get this error.
In the designer, select the Application (Database) in the outline (Application Outline).
Now click tools in the top margin.
Click "Recompile All Lotusscript"
As the name implies, all the lotusscript code present in the database will get recompiled.
It will show a progress info and after completion it will show the design elements where duplications or errors exist...
So u can rectify them.
Hope this helps :)