Error after update 4.5.3 => 5.1.1

[Remitur]

After update from 4.5.3 => 5.1.1, I'm getting following error accessing to /admin/configregistrars.php :

Error: Error: Call to undefined method WHMCS\Module\Registrar\Ispapi\Ispapi::checkAuth() in /var/www/domainregister.international/public_html/modules/registrars/ispapi/ispapi.php:792 Stack trace: #0 /var/www/domainregister.international/public_html/vendor/whmcs/whmcs-foundation/lib/Module/AbstractModule.php(0): ispapi_getConfigArray(Array) #1 /var/www/domainregister.international/public_html/vendor/whmcs/whmcs-foundation/lib/Module/Registrar.php(0): WHMCS\Module\AbstractModule->call('getConfigArray', Array) #2 /var/www/domainregister.international/public_html/admin/configregistrars.php(0): WHMCS\Module\Registrar->call('getConfigArray') #3 {main}

WHMCS 7.10.2
PHP 7.2.34

(after rolling back to 4.5.3 everything is fine)

[KaiSchwarz-cnic]

Hi @Remitur,

Some customers run into a similar issue as they installed the Source Code Package instead of the WHMCS Module Package. Both are listed under Assets in the Release Overview and therefore a potential reason for such issues.

Can you check this please?

Note: in the Source Code Package, the "lib" folder isn't included, so after Upgrade using that package, you either have an old version of the lib folder in use or no such folder. ... A bit stupid that github doesn't allow to deactivate source code archives in the release overview.

Ty!