Your first picture looks like the results from a Global Search (Ctrl+K). It often happens that a message appears in the list, but shows no content, if the search index hasn't been rebuilt since the message was deleted. Try rebuilding the database and see if the search results are the same.

https://support.mozilla.org/en-US/kb/rebuilding-global-database

Thanks for the answer. Unfortunately the messages that disappeared didn't reappear after rebuilding the database. The rebuilding of the database has indeed certified the loss of them in the sense that now the global search doesn't give any result. I was wondering if and how it could be possible that some messages that I sent, which are usually kept and found in the Gmail SentMail folder do disappear without me trashing them (which I never do). This is what happened to me in any case: some messages that I sent and that my correspondents have received, cannot be found anymore in the sent mail folder. Maybe it is a gmail problem, or my cell phone client problem, but I wonder if Thunderbird could be the cause of the issue. As I already said I am also missing some messages that I received and didn't send to trash, which is very annoying. Any feedback on this would be very much appreciated.

You can right-click the Sent folder, Properties, and check if the Retention Policy is set to delete mail after a certain time. If that isn't the cause, I can't say why your mail is missing. Perhaps search on the gmail site and see if the All Mail folder contains missing messages. TB should not be set to save sent message (uncheck 'Place a copy in' in Copies & Folders, Account Settings), as gmail automatically copies to Sent, so I suspect it is a gmail issue.

Hi, thanks again for the prompt answer. The retention policy is set to use the account settings, so no deletion planned. By the way, the missing messages are pretty recent and I can still find older messages in the sent items folder. Actually, only a few messages have gone missing, but that is concerning anyway. TB isn't set to save sent messages and the account settings are as per TB recommendations. I was just wondering if the the local folders on the PC where TB is installed got somehow corrupted and TB replicated the messages loss up to the mail server and then back down across my clients....

I don't think that Local Folders has any effect on IMAP mail folders, but you should check that you don't have any computers or devices that have the gmail accounts set up as POP, as that would affect syncing of the mail and could cause some devices to miss messages.

Thanks again. I don't have any device set on POP....Will post here if I can find the cause of the loss.