Trying to use an SPWeb object that has been closed or disposed and is no longer valid.
After reading this article:
I inspected my code and went from using this using (sorry had to write it like that):
using (SPSite spSite = SPContext.Current.Site)
and realized that I might be disposing the very site object that I'd need later on. So I changed it to this:
using (SPSite spSite = new SPSite(SPContext.Current.Site.Url))
and it now works nicely!