Error messages after upgrade to 2.9.1 [SOLVED]

In backend, when accessing the Site Structure of a multilanguage site, I get the error:

Code:

Recoverable error: Argument 4 passed to tl_page_changelanguage::addFallbackNotice() must be an instance of DataContainer, string given, called in /vhosts/hollandenvironment.com/httpdocs/system/drivers/DC_Table.php on line 3074 and defined in /vhosts/hollandenvironment.com/httpdocs/system/modules/changelanguage/dca/tl_page.php on line 192 #0 /vhosts/hollandenvironment.com/httpdocs/system/modules/changelanguage/dca/tl_page.php(192): __error(4096, 'Argument 4 pass...', '/vhosts/holland...', 192, Array) #1 /vhosts/hollandenvironment.com/httpdocs/system/drivers/DC_Table.php(3074): tl_page_changelanguage->addFallbackNotice(Array, 'Home', Object(DC_Table), 'style="margin-l...') #2 /vhosts/hollandenvironment.com/httpdocs/system/drivers/DC_Table.php(2803): DC_Table->generateTree('tl_page', '1', Array, true, -20, false) #3 /vhosts/hollandenvironment.com/httpdocs/system/drivers/DC_Table.php(334): DC_Table->treeView() #4 /vhosts/hollandenvironment.com/httpdocs/system/modules/backend/Backend.php(234): DC_Table->showAll() #5 /vhosts/hollandenvironment.com/httpdocs/contao/main.php(101): Backend->getBackendModule('page') #6 /vhosts/hollandenvironment.com/httpdocs/contao/main.php(304): Main->run() #7 {main} Warning: Cannot modify header information - headers already sent by (output started at /vhosts/hollandenvironment.com/httpdocs/system/functions.php:134) in /vhosts/hollandenvironment.com/httpdocs/system/libraries/Template.php on line 186 #0 /vhosts/hollandenvironment.com/httpdocs/system/libraries/Template.php(186): __error(2, 'Cannot modify h...', '/vhosts/holland...', 186, Array) #1 /vhosts/hollandenvironment.com/httpdocs/system/modules/backend/BackendTemplate.php(142): Template->output('Content-Type: t...') #2 /vhosts/hollandenvironment.com/httpdocs/contao/main.php(295): BackendTemplate->output() #3 /vhosts/hollandenvironment.com/httpdocs/contao/main.php(104): Main->output() #4 /vhosts/hollandenvironment.com/httpdocs/contao/main.php(304): Main->run() #5 {main}

It's sort of ruining my upgrade experience... anyone any expertise with this?