Dec 01, 2020
First off, awesome software at a very good price.
As for my issue, it's a couple of odd filenames, I'll give you that!
2020.12.01 02:41:25.907 (UTC-5) 2 2 17. Copying file Steam\steamapps\common\Proton 4.11\dist\share\default_pfx\dosdevices\c:
2020.12.01 02:41:25.907 (UTC-5) 3 3 28 bytes, created 2020.10.02 18:08:01.619, modified 2020.10.02 18:08:01.619, system archive
2020.12.01 02:41:25.907 (UTC-5) 3 4 Raw: 28 / 132461536816197845 / 132461536816197845 / 00000024
2020.12.01 02:41:25.910 (UTC-5) 0 3 CreateFile() failed with 123
2020.12.01 02:41:25.910 (UTC-5) 3 4 Context: \\?\G:\Steam\steamapps\common\Proton 4.11\dist\share\default_pfx\dosdevices\c: 80000000 00000003 00000003 4a000000
This is a steam backup. I understand that my destination (Linux server share through samba) can't write that filename ("c:"). What would be the best way to exclude that file only OR better yet, could there be a way to have a "flag" option allowing some files to be renamed to accommodate the backup destination (and of course reinstate the right filename upon restore)?
I can see that the latter would be a feature request - and a pretty big one - and I'm wondering if it would be worth it to add the complexity for the gain. I'll leave that analysis to you but figured I'd ask!
Thanks again, love the software and would recommend it to anyone without hesitation.
- A customer who can't live with errors in his logs!
Alex Pankratov :
Dec 01, 2020
In terms of the cause, you got it right - Error 123 stands for "The filename, directory name, or volume label syntax is incorrect." 
What is really strange is that something managed to create this file in the first place. Colon is a reserved symbol and cannot be used in the file/folder names . It's used for separating the drive letter or the alternate data stream name in a file. I can only guess that it got sneaked in via WSL or by mounting the volume using a non-native NTFS driver.
In any case, I'd say the right thing to do is to exclude this file from the backup. You should be able to do that by clicking on it in the mid pane in Backup Settings > Backup What. I don't remember offhand if there are any strict checks for the path names, so there's a chance that this won't work if there are any. In which case we can try and do it differently.
But, damn, if it's not a sign of some seriously hacky stuff going somewhere in the guts of this Proton thing.