When I log into the machine as ansible_user, I do not need to provide the credentials to access \\server1\share1\folder1 using File Explorer. However, when I run the playbook, i get an Access Denied error.
What authentication method are you using for WinRM? The method you select needs to support credential delegation to use UNC paths. This is not an Ansible problem but a side effect of WinRM and Windows. It can be replicated in a remote PSSession using powershell commandlets.
You are using become but with no become password. Set the ansible_become_password variable to {{ ansible_password }} (or whatever var contains the password. Keep in mind this is a variable not a task option so needs to be set under the vars key.
Here is my solution to the same problem:
The problem was something with that you need an extra login for windows hosts because how windows uses logons or whatever. Dont remember.