Blank page after login

After upgraging to 2.4.4 from 2.4.2 I have a problem. In my main administration after i enter my credentials i get a blank screen only with the footer show

--Source from webpage ---
<center><center><table width='20%' cellpadding='2' cellspacing='0'>
<tr>
<td align=center><a href='javascript:goBack();'>Terug</a></td>
</tr>
</table></center>
</center><br></div></div>
</div>
<div class='fa-footer'>
<span class='power'><a target='_blank' href='https://frontaccounting.com'>FrontAccounting 2.4.4</a></span>
<span class='date'>02/09/2018 15:28</span>
<span class='date'>JWBosch Techniek</span>
<span class='date'>jwbosch.nl</span>
<span class='date'>Jan Willem Bosch</span>
<span class='date'>Thema: canvas</span>
<span class='date'></span>
</div>
</div>
<script type='text/javascript'>
        _focus = '';
        _validate = [  ];
        var editors = [  ];
    </script>
<script type="text/javascript"><!--

var user = {
theme: './themes/canvas/',
loadtxt: 'Opvragen van gegevens....',
date: '02/09/2018',
datesys: 0,
datefmt: 1,
datesep: '/',
ts: '.',
ds: ',',
pdec : 2}

--></script>
</body></html>
--- End Source from webpage.

Here is what the error log shows :

[02-Sep-2018 15:28:15 Europe/Berlin] 0::session.inc:432:[before upgrade] ini_set(): A session is active. You cannot change the session module's ini settings at this time
[02-Sep-2018 15:28:15 Europe/Berlin] 0:jwbosch:session.inc:432:[before upgrade] ini_set(): A session is active. You cannot change the session module's ini settings at this time
[02-Sep-2018 15:28:15 Europe/Berlin] 0:jwbosch:dashboard.php:16: Unhandled exception [0]: Call to undefined method dashboard_app::application().

What must i do to see the mainscreen again?

J.W. Bosch

JWBosch Techniek
-- Work - Frontaccounting - terrace --