eu tive a mesma solução para problemas, execute-o com param:
env QT_AUTO_SCREEN_SCALE_FACTOR=0 /opt/viber/Viber
De link
Usando o Ubuntu 16.04 com todos os upgrades. O Viber parou de carregar após a atualização para a versão mais recente 6.5.5. Problema permaneceu após a reinstalação limpa do Ubuntu.
Abaixo está a saída de /opt/viber/Viber
/opt/viber/Viber
QSqlDatabasePrivate::removeDatabase: connection 'ConfigureDBConnection' is still in use, all queries will cease to work.
Qt WebEngine ICU data not found at /opt/viber/resources. Trying parent directory...
Qt WebEngine resources not found at /opt/viber/resources. Trying parent directory...
Qt WebEngine ICU data not found at /opt/viber/resources. Trying parent directory...
Qt WebEngine resources not found at /opt/viber/resources. Trying parent directory...
QXcbConnection: XCB error: 3 (BadWindow), sequence: 508, resource id: 54525979, major code: 2 (ChangeWindowAttributes), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 509, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 510, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 511, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 513, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 514, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 515, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 519, resource id: 54525979, major code: 18 (ChangeProperty), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 523, resource id: 54525979, major code: 2 (ChangeWindowAttributes), minor code: 0
................
Não é possível fazer o trabalho, não é possível encontrar a versão anterior que funcionou.
A ajuda será muito apreciada!