See After libxkbcommon update in fedora 34, insync no longer starts [with workaround]
Fix:
cd /usr/lib/insync
sudo mv libxkbcommon.so.0 libxkbcommon.so.0.bak
sudo ln -s /usr/lib64/libxkbcommon.so.0.0.0 ./libxkbcommon.so.0
then insync starts normally.