There is also OpenLyrics.
If you know that there are typical errors where the error can be replaced with the correct character, then it should be easy to create an action group that replaces all the errors with the corrected characters.
It would be up to you, though, to research these frequent errors.
What happens to the squares when you load that blue text directly into the lyrics editor?
This squares in the blue text are not even real special characters, only a wrong replacement for a space. Maybe something like a non-breaking space?
It would be useful to see this character in a hex editor.
I don't know what happens, I copy the text, I paste it in Notepa2, I don't touch anything, I just click on Encoding - UTF-8 with signature and then I copy the text and paste it in the flac and everything works fine.
If you tell me a way to send you the flac look at it
You could send a personal message...
What happens if you use the following action on the lyrics field:
Action of the type "Replace with regular expression"
Search string: \b
Replace with: _
(where the underscore is meant to be a space character but that would be invisible in this forum)
Try this on a test file.
It would be nice if the square would disappear in the displaying program.
the file is too big for the private message, I have post it here
https://www.transfernow.net/it/manage/20220830bJOJAR29/wbzyEQVw
With notepad2 it doesn't find \b even though I checked regular expression 
For me it looks like this squares are a U+2005
e2 80 85 FOUR-PER-EM SPACE
Yes, I also had a look at your file and the LYRICS field has some special Unicode spaces "Four-Per-Em Space (U+2005)"
You can use an action Replace with regular expression for field LYRICS with a
Regular expression: \x{2005}
and
Replace matches with: _ ← that should be a normal space character
This looks like a special space character -
You could remove any number of spaces with
Replace with regular expression for LYRICS
Search string: \s+
Replace with:_ ← that should be a normal space character
You could also filter for such files:
%lyrics% MATCHES \x{2005}
Thanks to all but ... now what do I do? 
Is there any way to create an automatic way to scan all flacs and fix errors?
I am a little disappointed as I just tried to show a way to look for files and clean them - didn't you read that?
But to get this totally clear: this will fix only this particular abnormity.
It could very well be that you find other problems.
But in general it means: you are probably not limited to the manual way but MP3tag has functions to help in a lot of files in a short time.
But I already clean them by converting them to UTF-8 with signature
My problem is not understanding what error there is in the texts but converting them all together
But you are already now aware that the problem was not simply UTF-8 encoding, but the included Unicode spaces "Four-Per-Em Space (U+2005)" from your examples.
This cannot be just assumed by mp3tag, as often there are Unicode characters intentionally used in fields, including lyrics. You will need to identify each case where there is another non-standard character that is affecting your player's ability to display these lyrics. The tools and suggestions throughout this thread should be sufficient for you to move forward from this point. Apply them to each case as required now, and for any new lyrics you attach in the future.
sorry but for me it is arabic I, when I convert the text to UTF-8 encoding, I don't remember ever having any problems.
Jokes? I don't ask for perfection, I ask if there is a way to solve most of the problems I have. And, your advice, it seems absurd to me if you have read my posts and understood my technical preparation. But I want to hope that you are kidding
I don't believe anything I said was in a joking manner, nor was it intended to make light of the challenges you are having to resolve your lyrics.
I was very directly saying that all of the previous suggestions here have provided you with a path to follow for some of these odd characters that are causing you grief. But it isn't realistic to expect that for every possible variation you may come across...
...that somebody else can search through the code to find the problem character, and the revisions to the Action required.
Similar to what was pointed out in your example file, you would have to look through any other files that are not working and find the problem character. Then update the Action posted above using that character instead. This is a very manual process, I realize. But since the ideal case of finding your perfect lyrics doesn't seem to exist, there is no better solution to offer.
sorry but, I repeat, I do not need anything what you wrote because I do not understand it and I am unable to do anything. It is as if a sick person enters a forum of doctors because he is not well and they advise him to study medicine to heal, I hope you are joking What you have written can be useful to you, those who are good and kind and who they have the preparation to help me, I can not help but describe the problem and thank you for your help.
The best solution is to find a good and kind one who can create a command that repeats what Notepad2 does as I have described several times. Nothing more and nothing less. I do not expect perfection but only that you recreate the same process that Notepad2 does and I believe it is possible
OK: what about you?
Or if you cannot do it but you have this problem because you use a certain workflow - what about changing that workflow?
Or what about some deeper research by you which tool besides MP3tag could serve your purpose?
Or what about a tool chain with MP3tag that exports the lyrics field to a file, calls your favoured Notepad2 for conversion, and an action that reimports the data?
This just to point out that there are still alternatives.
An, honestly, repeating the same stuff over and over again while others strain their brains to deal with your problem (the source of which is probably that 3rd party program that you refuse to give up), will not bring you 1 step closer to a solution.
The options that you have with Mp3tag lie on the table. If you are not satisfied with it, then I fear that not a lot can be done about it.
I doubt that an emulation of Notepad2 (?) will be implemented very soon. So, until then you either have to live with the limitation or try to get some help with already existing functions in MP3tag.
ok, you are right, thanks for your help Hello