Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the following as you can (you can cut and paste this into your topic):
Nextcloud version (eg, 10.0.2): 12.0.3
Operating system and version (eg, Ubuntu 16.04): 16.04
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.10.3
PHP version (eg, 5.6):
Is this the first time you’ve seen this error?:
Yes
Can you reliably replicate it? (If so, please outline steps):
Just let the OnlyOffice VM run
The issue you are facing:
Nextcloud can communicate with OnlyOffice when accessing Nextcloud from within the LAN. If I acccess Nextcloud outside of the LAN, I cannot access OnlyOffice.
Nexcloud runs on VM1, OnlyOffice runs on VM2 and Nginx runs on VM3. Nginx functions as a reverse proxy for all the services I run inside my home network. It proxies requests at cloud.example.com to nextcloud. Next cloud is connected to OnlyOffice using the LAN IP in the admin settings.
Do I need to proxy OnlyOffice and provide a fully qualified domain name in order to use it outside of the LAN or is there something else I’m missing?
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.