The SFP's of Siqura are designed for fibers with (U)PC endface: (Ultra) Physical Contact.
When you have a fiber with APC endface, Angled Physical Contact, you need a patch cable from APC to UPC. The APC endface towards the fiber and the UPC endface to the SFP.
The main difference between APC and UPC connectors is the fiber endface. APC connectors feature a fiber endface that is polished at an 8-degree angle, while UPC connectors are polished with no angle. UPC connectors are not exactly flat however; they have a slight curvature for better core alignment. Another more obvious difference is color. UPC adapters are blue while APC adapters are green.