Dear All, I am unable to create a subfolder that has a very similar name to the folder above it. The folder structure has 6 levels so far; the structure below shows only 2 levels for demonstration purposes (I'd be happy to share the full structure with the correct names somewhere privately if that would assist with investigating the issue). e.g. 01 Topfolder 1234 --12-34, Testfolder XYZ In this last folder I would like to create the folder '56-78, Testfolder XYZ' I do this via rh click on 12-34 Testfolder XYZ, selecting 'New subfolder'. The 'New Folder' box appears prompting for a 'Name'. When I enter '56-78 Testfolder XYZ', and hit 'Create Folder', the box just stays there without the folder being created. I can click 'Cancel' and start the process again and choosing a completely different name, eg 'a'. This then generates a folder named 'a' on the 7th level (of my real folder structure). When I’m checking in C:\UsersMyName\AppData\Roaming\Thunderbird\Profiles\l5yq8b1p.default\Mail\Local Folders\01 Topfolder 1234\12-34, Testfolder XYZ: here I can see only the MSF and mBox files for folder 'a' but not the '56-78, Testfolder XYZ', so I assume it has not been created. There is yet another detail to this problem - there used to be a subfolder, called '56-78 Testf', on the 7th level (of my real folder structure) containing 3 mails. It froze with the eggtimer showing when I tried to rename it. I then deleted it and checked it had disappeared from the Windows file system as well. Also I compacted all the folders in the hierarchy and repaired the last folder in the line, hoping this would make a difference and restarted PC and Thunderbird several times. Another detail - I was working mainly offline when dealing with this issue. I work with Win 10. OK, I can live with a subfolder in the seventh level simply called 'a' and move the 3 mails from my backup into it but still, perhaps someone could shed some light on this. Regards Blackredgold1964