Cannot import title without number from filename

Up to tonight, I was able to remove the leading digits from file names like 210710.0530 Conserving small things on a big scale .mp3 by using MP3Tag, but suddenly it no longer works.
What can I do to make it functional again ?

Good question.
As you hardly say anything about the functions that you use or the (real) names, it is almost impossible to get any constructive ideas.

I said that it used to strip the leading digits off a file like '210710.0530 Conserving small things on a big scale .mp3' - that's what I use it for, so that I would end up with the name being 'Conserving small things on a big scale .mp3

OK, and which function do you use?
Converter>Filename-Filename?
Format value?
Converter>Tag-Filename?
Replace with regular expression?

Select the files; Convert menu; Filename-tag; 'title' from the dropdownlist ; OK

Ah - this copies everything from the filename to the field TITLE
I am sure, that you find your original pattern in the dropdown list.
this may look like
%dummy% %title%
or
%track% %title%

I don't understand that answer. I can not reproduce the faulty phenomenon, until I get another download whose name is preceded by a string of digits.

Where do you see a fault in MP3tag? If you provide just a single field variable like

(which should be %title%)
then everything from the filename is copied to the field TITLE.
If you want to split the filename, then you have to tell MP3tag what you like and what you want to discard.
The dropdown list labelled "Format string" is filled with the strings that you entered previously ... this is no MP3tag preset. So it is fairly likely that you find the old, working format string there somewhere.
And ... if MP3tag imports data from the filename then it is still working. That the result is not as you expected ... well, that is something different.

It's not that my expectation is disappointed, but that the routine procedure I used for years, no longer worked.

So, what was the result of your check of the dropdown list for format strings in Converter>Filename-Tag?
Did you find a format string that shows more than just %titlle%?
Again: if you use just %title% as format string, then all the data from the filename is copied to the field TITLE - and nothing gets eliminated (except the file extension). So it is obvious that you cannot have used just %title% to get a smaller part of the filename into TITLE, esp. without the leading number.
From this point onwards it is pointless to try to identify any kind of culprit.
If you still want to import data from the filename except a leading number or string, I would suggest either of the following:
Converter>Filename-Tag, Format string: %dummy% %title%
An action of the type "Format value" for TITLE, format string: $regexp(%_filename%,\d+ (.*),$1)
An action of the type "Guess value", source: %_filename%, target: %dummy% %title%

All these actions have the advantage that they are saved permanently and do not get lost during an interactive session. I would consider this the safer way.

Thank you for your detailed info. I will try your method, next I get the chance.

You know that Converter>Filename-Tag has a preview.
So if you enter %title% as format string for any file, then you should see something in the preview.
If you now append the format string to %dummy% %title% then you should see that in the preview the first part up to the first blank is missing. This could indicate that this string will work also for leading numbers, separated by a blank from the rest.

It just came good by itself, so there is no problem to fix.

What does the format string look like now?

[number code][filename] goes in, [filename] comes out.

The syntactical correct format string features string constants and MP3tag property variables, like %title% or %artist%
So, what does this format string look like?

Input is %number% %filename%; output is %filename%

Like that you create 2 user-defined fields: one called NUMBER, the other one FILENAME.
You can check that in the extended tags dialogue (Alt-T).
If you want to address the filename, then the correct variable would be %_filename% even though I doubt that you can rename a file with this converter. The user-defined field NUMBER stays, though.