Trouble adding virtual host (no replies)

It has been a while since I created a virutal host and I'm having problems.

I just installed the newest release of WAMP64 on a computer which has never had WAMP on it.

I am getting the following messages when I try to create a virtual host. I assume the 2nd and 3rd errors are due to the open failure.

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Warning: fopen(C:/Windows/system32/drivers/etc/hosts): failed to open stream: Resource temporarily unavailable in C:\wamp64\www\add_vhost.php on line 446
Call Stack
# Time Memory Function Location
1 0.0000 410920 {main}( ) ...\add_vhost.php:0
2 0.0156 486824 fopen ( ) ...\add_vhost.php:446

( ! ) Warning: fwrite() expects parameter 1 to be resource, boolean given in C:\wamp64\www\add_vhost.php on line 447
Call Stack
# Time Memory Function Location
1 0.0000 410920 {main}( ) ...\add_vhost.php:0
2 0.0156 487440 fwrite ( ) ...\add_vhost.php:447

( ! ) Warning: fclose() expects parameter 1 to be resource, boolean given in C:\wamp64\www\add_vhost.php on line 484
Call Stack
# Time Memory Function Location
1 0.0000 410920 {main}( ) ...\add_vhost.php:0
2 0.0156 487168 fclose ( ) ...\add_vhost.php:484
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I checked and the hosts file is located at C:\Windows\System32\drivers\etc\ and I can open it in an editor

The virtual host I'm trying to create resides in: C:\wamp64\www\WP

I've tried different virtual host names with no difference in results - I followed the rule "No diacritical characters (éçëñ) - No space - No underscore(_)"

I've tried running WAMP64 normally and I also ran it As Administrator.

What am I doing wrong?

What can I do to deleted the partially defined virtual hosts?