Joomler, thanks again for your patience, I've waited so long because I wanted to monitor more precisely the bahaviour of the system.
It revealed to be very strange: sometimes it work without problems, then stops giving errors, and it resumes without touching anything.
I've checked my PHP settings, and they aren't like yours...I know this should be an issue, but I'm on a shared hosting, and don't believe they are going to change settings...
What do you advise me to do?

ICONV
iconv support enabled
iconv implementation glibc
iconv library version 2.3.4

Directive Local Value Master Value
iconv.input_encoding ISO-8859-1 ISO-8859-1
iconv.internal_encoding ISO-8859-1 ISO-8859-1
iconv.output_encoding ISO-8859-1 ISO-8859-1

MBSTRING
Multibyte Support enabled
Japanese support enabled
Simplified chinese support enabled
Traditional chinese support enabled
Korean support enabled
Russian support enabled
Multibyte (japanese) regex support enabled

mbstring extension makes use of \"streamable kanji code filter and converter\", which is distributed under the GNU Lesser General Public License version 2.1.

Directive Local Value Master Value
mbstring.detect_order no value no value
mbstring.encoding_translation Off Off
mbstring.func_overload 0 0
mbstring.http_input pass pass
mbstring.http_output pass pass
mbstring.internal_encoding ISO-8859-1 no value
mbstring.language neutral neutral
mbstring.substitute_character no value no value