You'll need to check with your web host on how to give php more memory. It can be done several different ways and every host is different. Some let you have your own php.ini files, some will have to make the change for you, and some won't budge. Check their support documentation or contact them directly.

My b2evolution version is 1.8.2.

I have made some changes on my php.ini file, but I think that it's not this... I have read again, the "error description" and in the file _plugin.foncs.php the line 362 is this one :

{
$param_error = sprintf(T_('The value is invalid. It must be in the range from %s to %s.'), $l_meta['valid_range']['min'], $l_meta['valid_range']['max']);
}

So, I suppose that I need to change the "valid range" but I don't know where... :-/

The only "valid range" available on the php.ini file are these ones :

; Valid range 0 - 2147483647. Default = 4096.
;mssql.textlimit = 4096

; Valid range 0 - 2147483647. Default = 4096.
;mssql.textsize = 4096

Help... :'(

I don't think there's anything wrong with your php.ini or the php file _plugin.funcs.php. The valid ranges you mentioned have nothing to do with each other. The main problem is that your server has only allotted 8 MB of RAM for php to use. Apparently that's not enough. Are you getting this error consistently? Are you getting it anywhere else besides the plugin page? If you remove the youtube_plugin folder, does the error go away?

Thanks for your quick answer!
Yes if I delete the YouTube Plug-In the "error" will not appear...

So I currently check again the php.ini file.

Thanks ;)