The following warning has been displayed since the last update. What can I do about this?
First one, can you clocky on documentation link and post the page link here?
For the second one can you run this using SSH:
snap run nextcloud.occ maintenance:repair --include-expensive
Can you try
snap run nextcloud.occ db:add-missing-indices
Created an issue to package it: Nextcloud Talk - High performance backend · Issue #727 · syncloud/platform · GitHub
What kind of issues do you get besides the red warning right now?
The PHP OPcache module is not properly configured. The OPcache interned strings buffer is nearly full. To assure that repeating strings can be effectively cached, it is recommended to apply “opcache.interned_strings_buffer” to your PHP configuration with a value higher than “16”.. For more details see the documentation 
.
I mean do you have problems because of the Nextcloud Talk warning?
No, that’s not a problem for me.