Using P3D 4.5 under Windows 10 shouldn't have many issues of missing runtimes or DLLs, since it uses the same runtimes as the sim, and doesn't rely on older stuff from FSX so, it's now looking more like an antivirus issue.
Please be sure you add the whole Addon Manager folder to the antivirus Exclusions. DO NOT "turn off" the antivirus, we NEVER suggest to do that, most antivirus are not really turned off, unless you uninstall them, because they low-level service is still running, that's why we never say to "turn off " the antivirus but, leave it on, and configure it properly, by adding the Addon Manager folder to the antivirus Exclusions.
Adding the whole simulator folder to the Exclusions will also help for performances and, if you keep your add-on sceneries on a different drive/folder than the sim, add those to the Exclusions too.