We have identified an issue in 2.1.8 that may cause certain template modifications in add-ons to not be applied correctly. This issue is discussed in more detail in this bug report. In order to resolve this, we have released XenForo 2.1.8 Patch 2.
XenForo 2.1.8 Patch 1 Released

We have fixed two issues in XenForo 2.1.8 which cause errors or unexpected behavior:
We have released XenForo 2.1.8 Patch 1 to resolve these issues.

You can perform the upgrade directly from your control panel by going to Tools > Check for upgrades. You can also download the update from your Customer area.