@lah7 many thanks first of all. The solution you provided is in part right but need to be completed further.
To fix temporarily the issue, I replaced “Exec=synapse” in
Exec=bash -c ‘GTK_IM_MODULE="" synapse’
and it works now.
If I want synapse to open at login I have to add:
bash -c ‘GTK_IM_MODULE="" synapse’
as a command in Startup Applications (eventually also in the Main Menu for when it is launched via the Applications menu).
This work around is described in https://bugs.launchpad.net/synapse-project/+bug/1523279
Again many thanks for your support.