We play on a server with friends with a collection of all of fabrick 7 mods. When we open chests in the fortress in hell or Warden, loot does not spawn, but loot appears in other chests
latest.log
https://drive.google.com/file/d/1pPJXAPk5LA-kRP3ozOj_xSg_Pch2UN_q/view?usp=drive_link

Generally speaking, when these chests open but are empty, there is an issue with the loot table, and Lootr will generally report as such. If you look through the log, you can see the following messages:

[19:33:17] [Render thread/INFO]: [CHAT] The loot table for this container [minecraft:chests/ancient_city] resolved to empty and loot could not be generated. Check your `latest.log` for errors.
...
[19:38:18] [Render thread/INFO]: [CHAT] The loot table for this container [minecraft:chests/ancient_city] resolved to empty and loot could not be generated. Check your `latest.log` for errors.
...
[19:47:15] [Render thread/INFO]: [CHAT] The loot table for this container [minecraft:chests/ancient_city] resolved to empty and loot could not be generated. Check your `latest.log` for errors.
[19:54:28] [Render thread/INFO]: [CHAT] The loot table for this container [minecraft:chests/ancient_city] resolved to empty and loot could not be generated. Check your `latest.log` for errors.

However, this latest.log file appears to be a client-side log. I'd recommend check the server's latest.log file to see if there's any mention of minecraft:chests/ancient_city being invalid or failing to load.

With that said, if you aren't modifying any of these loot tables yourself, this would be something to take up with the designers of the mod pack.

I've had a quick look through their issue tracker and found: TeamAOF/All-of-Fabric-7#9

It specifically includes the ancient_city loot table; it appears to be fixed upstream but perhaps the modpack itself has not updated. I would recommend checking for updates to the pack and otherwise replying to that issue to bump it.