After upgrading a SOGo installation from 3.1 to 4.0.0, reading emails still works perfectly. But writing (or replying) is broken: I can enter recipient, message, Cc etc. as usual, but it can't seem to figure out what my sender address is. Instead, I see a dropdown box for "From:", but opening it shows an empty box instead of possible sender addresses. Sending the message is disabled (probably because From: can't be blank, but there's no way to make it non-blank).

SOGo does know who I am because my correct name and email address and gravatar icon are displayed on the top-left in the mail reading interface.
Adding another IMAP account doesn't help.

Users are stored in a MariaDB database.

    SOGoUserSources =
    (
            {
                    type = sql;
                    id = directory;
                    viewURL = "mysql://****:****@localhost:3306/mail/users";
                    canAuthenticate = YES;
                    isAddressBook = YES;
                    userPasswordAlgorithm = sha256;
            }
    );

I don't see anything suspicious in the log.

May 13 16:13:51 sogod [5007]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail HTTP/1.1" 302 0/0 0.053 - - 0
May 13 16:13:51 sogod [5007]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail/view HTTP/1.1" 200 18594/0 0.180 78508 76% 0
May 13 16:13:51 sogod [5007]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Calendar/alarmslist?browserTime=1526220831 HTTP/1.1" 200 63/0 0.022 - - 0
May 13 16:13:51 sogod [5007]: <0x0x55a1dfde7c20[NGImap4Client]> TLS started successfully.
May 13 16:13:52 sogod [11863]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail/0/view HTTP/1.1" 200 914/0 0.311 - - 0
May 13 16:13:52 sogod [5007]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail/1/view HTTP/1.1" 200 914/0 0.408 - - 0
May 13 16:13:55 sogod [11863]: 185.184.176.69 "POST /SOGo/so/bero@lindev.ch/Mail/0/folderINBOX/view HTTP/1.1" 200 279584/48 2.956 - - 1M
May 13 16:13:56 sogod [11863]: <0x0x55a1dfa80e60[NGImap4Client]> TLS started successfully.
May 13 16:13:56 sogod [11863]: 185.184.176.69 "POST /SOGo/so/bero@lindev.ch/Mail/unseenCount HTTP/1.1" 200 50/47 1.432 - - -1328K
May 13 16:14:03 sogod [11863]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail//UIxMailPopupView HTTP/1.1" 200 14456/0 0.184 57878 75% 0
May 13 16:14:03 sogod [11863]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Calendar/alarmslist?browserTime=1526220843 HTTP/1.1" 200 63/0 0.016 - - 0
May 13 16:14:03 sogod [11863]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail/0/compose HTTP/1.1" 201 78/0 0.228 - - 0
May 13 16:14:03 sogod [11863]: 185.184.176.69 "GET /SOGo/so/bero@lindev.ch/Mail/0/folderDrafts/newDraft1526220843-1/edit HTTP/1.1" 200 92/0 0.220 - - 0