Hi Fabian,

It took me some time to find out what's wrong, and the good news is, it's not your fault! :) The bad news, of course, is that this is a platform bug. I've encountered something similar it myself some time ago, but this had to do with decimals instead of Ids, so I didn't immediately make the connection.

To find the cause, I had to manually alter the C# code the platform produces and add some Audits. The problem is that Session Variables are stored in an associative array of variable type. When the screen starts, it's type is (correctly) System.Int64, as your identifier is a Long Integer. However, when you assign NullIdentifier to it, the platform assigns BuiltInFunction.NullIdentifier() to the session variable, but that seems to return a System.Int32, in effect changing the type of the session variable to System.Int32. When you do the Ajax refresh, the platform calls a function called selcbxSourceFileNames_onDataBinding (you can see that name appearing in the Service Center error log stack trace), which tries to cast the session variable to a long, which is invalid, and you get the cast error.

So, what to do? As a workaround, you could use a local variable instead of a session variable for the combo box, and instead of assigning NullIdentifier() to the session variable, assign it to the local variable, then assign the local variable to the session variable (and don't forget to initialize the local variable in the preparation by assigning the session variable to it). Or, you could (depending on the stage of development you're in) change the identifier of the table from long int to int, since it doesn't seem you'll have billions of records in that table anyway (note that in order to do this, you need to Ctrl-X/Ctrl-V the table, and you'll lose all the data currently stored in it, as you cannot convert from int to long int, at least not in MSSQL).

I'll report this issue to OutSystems Support, so they can fix it forthwith. Hopefully they'll fix it soon.