Dear Community, thank you for this great piece of software like Thunderbird or Firefox! This question was asked quite some times, but I never found a definitive answer. I have the Thunderbird "profiles folder" on an SMB NAS folder, which is either accessed by a Thunderbird Windows 10 PC or by a Thunderbird running on Ubuntu Linux. Both Thunderbirds are configured to use the the same profile folder for each email account saved. The following happens: 0. I create a backup of the Thunderbird profiles folder 1. I am starting Thunderbird on Windows. This works fine, since this is my main machine and used regularly. 2. I close Thunderbird on Windows 3. I open Thunderbird on Linux. All the Email accounts are listed correctly and the google calendar access works fine. On the Home-Tab, the message "Unable to open the summary file for Inbox. Perhaps there was an error on disk, or the full path is too long" is displayed and I cannot access the emails. The length of the path is 111 characters long, so I think it should be fine. Also the Linux has complete acces to the NAS folder. 4. I close the Thunderbird on Linux 5. I open the Thunderbird on Windows. All messages from all Email accounts are downloaded again by the Thunderbird on Windows. 6. To skip this, I close the Thunderbird on Windows and go back to the backup from "0.". When I restart Thunderbird on Windows, Thunderbird start as expected. My question: How do I use two Thunderbirds with one profile folder to avoid having a ~20 GB profile folder on each machine? What settings could I try on the Linux Thunderbird so it can access the summary file? I am happy for any hints :) Have a great day