For all Revit versions, and especially Revit 2018, you must update to the latest point release (Revit 2018.3.3) to continue to use BIM 360 Design and earlier versions of Revit cloud worksharing.
A security fix must be applied to Personal Accelerator (PAC) on each workstation. Personal Accelerator for Revit is an application which is critical for the performance of the Revit cloud worksharing and cloud models for Revit functionality and serves all versions of Revit (2015-2020) on a single workstation. A Personal Accelerator security fix is part of several product updates. Refer to the link below.
To apply this Revit Security Fix, use your Autodesk Desktop App located at the bottom right of your screen (in the hidden icons).
If you use Revit cloud worksharing with Revit 2015 or 2016, you must also apply the latest Autodesk Licensing Manager hotfixes listed in this Autodesk Knowledge Network article: https://knowledge.autodesk.com/support/revit-products/downloads/caas/downloads/content/autodesk-transport-layer-security-updates-require-revit-security-fix.html
Whether your team needs assistance with updates, security fixes, worksharing or other productivity issues, contact the industry-trained experts at Applied Software. They’ll help you work through your challenges and get back on track.