When adding a certificate from Certum you may find that the chain certificate is not issued within the zip containing the certificate file like most other providers. If you ordered a DV certificate please use the below as your ca bundle.

https://repository.certum.pl/bundles/DVSSLChain-SHA2.crt

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----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=
-----END CERTIFICATE-----


For OV certificates:

https://repository.certum.pl/bundles/OVSSLChain-SHA2.crt

and for EV certificates:

https://repository.certum.pl/bundles/EVSSLChain-SHA2.crt