Ok, I fixed it. Here's the explanation in case someone needs it in the future:
After narrowing out the causes (as seen in my edits), I figured out that the problem must be related to my CustomRoleProvider.
That class has methods like this one:
public override string[] GetRolesForUser(string Username)
{
List<string> roles = new List<string>();
using (DBEntities _db = new DBEntities())
{
try
{
var dbRoles = from r in _db.UserRole
where r.Users.Username == Username
select r;
foreach (var role in dbRoles)
{
roles.Add(role.Role.Name);
}
}
catch
{
}
}
return roles.ToArray();
}
So I was catching an exception and not doing anything with it. I removed the try-catch block, and got this message:
There is already an open DataReader associated with this Command which must be closed first.
A bit of stackoverflowing and I found this: There is already an open DataReader associated with this Command which must be closed first
Turns out my local connection string had MultipleActiveResultSets=true
but the connection string on my publish settings didn't. I modified my publish settings and voilà, seems to be working now.
I don't really know the advantages/disadvantages of having that setting, but it is working and I really need to move on. Thank you all for your help anyway.