Also, this change you made to the culture setting may cause null reference exceptions.
<globalization
culture="es-ES"
uiCulture="es-ES"
requestEncoding="utf-8"
responseEncoding="utf-8"
fileEncoding="iso-8859-15" />
It may cause null exceptions because the resource files for es-ES are not complete, they have missing keys. The default culture should not have any mising keys. I recommend change it back to this:
<globalization
culture="auto:en-US"
uiCulture="auto:en-US"
requestEncoding="utf-8"
responseEncoding="utf-8"
fileEncoding="iso-8859-15" />
auto means use the browser preferred language but for missing keys use the default culture en-US. en-US is the only resource that is always up to date. So if your browser preferred language is set to es-ES, then it will use the Spanish resource files but if any of the keys are missing it will fall back to the en-US files.
Hope it makes sense.
Best,
Joe