Since I am new and can't post links, you can try the following before resetting your phone.
DISCLAIMER: This is taken from an online article and I have not tried nor do I know the effects of these methods.
Methods to Fix Error Code 805a8011 in Windows Phones:
Here are three methods to fix or solve error code 805a8011 in Windows phones. Check these methods one-by-one and apply them until the problem gets solved.
This Problem is mostly caused by ?LOW STORAGE? space on your phone.
Delete some Photos, Music or Videos to make some space.
For OS to work properly, it need some free space. So what you have to do is to free some space by deleting Photos, Music and other stuff. By this you can successfully fixed error code 805a8011 in Windows Phones. Atleast make 100MB or more of free space on your phone. If the problem is not resolved follow the next method.
Try to manually synchronize your Microsoft account:
Go to ?Settings?.
Tap on ?email + accounts?.
Tap and hold on ?Microsoft account?.
Tap on ?Synchronize?.
After Synchronizing error code 805a8011 in Windows Phones will be solved. Also, Check if ?date and time? setting are correct and change them for manual. If the problem is not resolved follow the next method.
At first, connect you windows mobile device to a Wi-Fi or internet data.
Go to Mail.live.com and create a new Microsoft Live account.
Now, open ?Settings?. Tap on ?Emails + Accounts?.
Now, Tap on Add ?email?.
Now, select Microsoft account and type your Microsoft account?s credentials (Username and Password).
This will fix error code 805a8011 in Windows Phones. You can now install or update the apps on your Windows Phone easily. If the problem is not resolved follow the next method.