Ok so this is for people who when they try to log onto opticraft it lets them in then quits. I had this problem and this is how I fixed it.
Note: This can take quite a long time so be patient!
Step 1: Find .Minecraft folder and delete it and then empty recycle bin make sure it is gone!
Step 2: Download Advanced System Care 5 Just the free version. You can download here-
http://advanced-systemcare.soft32.com/free-download?gclid=CJO8j7f8y7ECFU6JfAod3VQATA Step 3: Click on Deep Care and do Scan and Repair.
Step 4: Now go and eat cookies or chat on forums while it does that!
Step 5: Once that's done close System Care.
Step 6: Now re download the minecraft launcher and log in.
Step 7: You will need to put opticraft back into your server list.
Step 8: Get on and have fun!
Note: You do not need to delete the minecraft launcher if you didn't delete it skip step 6.
Warning: Deleting .Minecraft will delete your texture packs, mods, and single player worlds so back them up!
Warning: Deleting .minecraft will also make minecraft forget your name and pass for login section make sure you know your password!
Note: Need help finding .Minecraft its easy ( If your using Windows If you are not you will need to google it ) click on the windows icon at the bottom left corner of your screen. Now type into the search bar %appdata% Now click on roaming. You should be able to delete .Minecraft from there.
Hope this help anyone having this problem if you have any question leave a reply!