New behavior after some action or convertion.

Example: after renaming hundreds of files, Mp3Tag tells me that it can not rename 15 files because it has filename conflicts ... but it does not show me those files. So I have to look in hundreds of files that have some kind of problem.
I suggest a way to show these files ... perhaps by changing the color of the selection or better yet, including a bold symbol in front of these lines, in a specific column.
I believe that manual selection and deselection in this column will also be required.
Thanks in advance.

You can set in Tools>Options>Messages which type of messages you want to get.
(i usually get a list of files that could not be renamed - so check your settings.)

If you apply a naming scheme that relies on string constants and field contents, it should be easy to apply a filter that shows those files that do not match that naming scheme.
I doubt that highlighting such files is really comfortable if you have to scroll through a list with hundreds of entries.

I do not just think about highlighting a file.
The idea would be a way to select and work only on those files.
Changing my naming scheme is not an option at this time. I have about 1 million audio files ... more including optical media.

I see this problem often and agree that it can be annoying. The message popup has microscopic type and the file names may not fit inside it. A colored symbol in an Mp3tag column would be nice because we could then sort on that symbol, which would group the skipped file names together. However, I agree with Ohrenkino that highlighting such files is not a good solution. Perhaps Florian will consider logging the list of files skipped to a text file that could be opened in Notepad for easy reference.

I see two variations of the problem during tag-to-file name conversions. The first is due to files that are truly duplicates (i.e., the exact same recording). The second is due to files that have the same artist/title/year tags but are alternate takes of the same song. Often these were issued on the same CD so the album name is also the same.

For the alternate takes, I have a workaround. My new file names are usually just %artist% - %title% - %year%, so I created a custom tag called TAKENUM to hold the song take number or letter. Then I added this field to the tags panel and modified my tag-to-file name conversion formula to add the parenthesized take number to the title, which will usually make it unique:

%artist% - $ifgreater($len(%takenum%),0,%title% '('take %takenum%')',%title%) - %year%

Translation: If field %takenum% has an entry, prefix it with the word "take" and a space, enclose the added value in parentheses, and append it to the title. Otherwise, simply use the title value alone.

When a take number is present, the file names look like this:

Cannon's Jug Stompers - Going to Germany (take 2) - 1927.m4a

If you prefer to add take numbers to your titles before the tag-to-file name conversion, you can do that with a simple tag-to-tag conversion:

Field: TITLE
Format String: %title% (take %takenum%)

Either way the take numbers have to be entered in the tags panel before running the conversions. For me that's not a big problem since the number of titles with alternate takes is usually small.

Did you know that you could shorten the lengthy $ifgreater() statement with [ and ]?

So
%artist% - $ifgreater($len(%takenum%),0,%title% '('take %takenum%')',%title%) - %year%
would become:
%artist% - %title%[ (take %takenum%)] - %year%

I did not know that so thank you, Ohrenkino!

The funny thing is that for many years I have used brackets that way in Winamp for "Advanced Title Formatting" but it did not occur to me to try that syntax in Mp3tag. Not only is it shorter but it makes the line easier to read.