Identified - We’re currently impacted by a Facebook bug, preventing some users to mention a Facebook page for the last 4 days:
https://developers.facebook.com/support/bugs/501315857001079/?disable_redirect=0

That's why you might receive the error message “Internal server error" trying to mention a page within our publishing tool.
We're very sorry for the inconvenience: we will update this incident as soon as we have more information from Facebook.
Oct 8, 11:04 CEST
Manager   ? Operational
Facebook Publishing & Data Processing   ? Degraded Performance
Twitter Publishing & Data Processing   ? Operational
Instagram Publishing & Data Processing   ? Operational
Linkedin Publishing & Data Processing   Operational
Google Plus Publishing & Data Processing   Operational
Youtube Data Processing   Operational
Facebook Timeline Contest   ? Operational
Facebook Page Barometer   ? Operational
Web site   ? Operational
Cloud Provider   ? Operational
Operational
Degraded Performance
Partial Outage
Major Outage
Maintenance
Manager - Response time
Fetching
Facebook Page Barometer - Response time
Fetching
Facebook Timeline Contest - Response time
Fetching
Past Incidents
Oct 20, 2018

No incidents reported today.

Oct 19, 2018

No incidents reported.

Oct 18, 2018

No incidents reported.

Oct 17, 2018

No incidents reported.

Oct 16, 2018

No incidents reported.

Oct 15, 2018

No incidents reported.

Oct 14, 2018

No incidents reported.

Oct 13, 2018

No incidents reported.

Oct 12, 2018

No incidents reported.

Oct 11, 2018

No incidents reported.

Oct 10, 2018

No incidents reported.

Oct 9, 2018

No incidents reported.

Oct 7, 2018

No incidents reported.

Oct 6, 2018

No incidents reported.