Ok the htaccess file looks fine. Nothing suspicious, malicious or invalid. Do these steps below. If the problem occurs again then your host probably does not allow some or all of the code in the BPS root htaccess file and you will have to contact your host, send them the BPS root htaccess file and ask them what they do and do not allow.
Manual Setup Steps
htaccess Core htaccess Files Manual Setup Steps:
1. Click the Create default.htaccess File button on the Security Modes page.
2. Click the Create secure.htaccess File button on the Security Modes page.
3. Select the Activate Root Folder BulletProof Mode Radio button and click the Activate|Deactivate button on the Security Modes page.
4. Select the Activate wp-admin Folder BulletProof Mode Radio button and click the Activate|Deactivate button on the Security Modes page.