When an Individual license of ExtendSim is activated, it creates a license file (extendsim.lic) that is stored by default in the end user's Documents\ExtendSim folder. If that license file becomes inaccessible, ExtendSim will not launch but will instead give an error message. The two most common reasons why the license file would suddenly not be accessible to ExtendSim are:
The license file got deleted. This can happen if the device is "swept" and any files that were created after installation get removed. The fix is to set the sweep such that it does not remove any .lic files.
The license file was modified, such as being changed to "extendsim.lic.bak". This can happen if the ExtendSim folder containing the license file was stored in the cloud (for example, on OneDrive) and then used to replace the ExtendSim folder on a device for another license. There is no need to store the files that ship with ExtendSim in the cloud. For OneDrive, you can exclude the license file or the ExtendSim folder from being synced with your device.