[zephyrot]

Thanks for your suggestion, Idolse.

By the way, I forgot to mention before that RTF file used "Verdana" text
and I made a second batch of tests like first post with "Times New Roman", which is supposed to contain stressed vowels and "ñ".

I have converted the RTF file to filtered HTML, I have modified the HTML file charset to "UTF-8" from the previous "windows-1252".

After that, I have executed again the conversion process two times,
one setting a blank on Calibre Input Codification field, and another one setting manually setting "UTF-8", but with both cases, all i get is strange characters like "viéndolo" instead of "viéndolo" and "sueños" instead of "sueños".

If nobody gives me more suggestions to test (and I will be more than happy to test a bit more before throwing the computer through the window), I will open a bug report.

I do not want to open a new bug report yet just in case someone knows a silly quick solution posted in another place that makes me feel like an idiot for not searching harder.