This is almost always because either you don't have permission to the Ephinea folder you're writing to, or your antivirus is blocking the update.
If you installed the game to a place like "C:\Program Files\EphineaPSO", Windows could be blocking the update. Probably better to have installed it into like "C:\Users\YourName\EphineaPSO" so the launcher doesn't need admin permission to update/write files.
*IF* this is the case, you can run online.exe as an Administrator by right clicking online and telling Windows to run online as admin to see if it helps your problem.
If this isn't the case, please be sure you've whitelisted the Ephinea folder with whatever antivirus solution you're running on your machine. Antivirus solutions are notorious for claiming Ephinea as a false positive.
If either of those don't work, check your hard drive space remaining. Do you have any?