Hey yo on my cauldron or thermos server comes with the command / reload:
Reload not allowed on a cauldron server
How can I fix this?
Probably not at all. Presumably, the modifications do not cope with reloads.
But my hexxit 1.5.2 server is ok (but bukkitforge is not available in 1.7.10 anymore
Is probably because there are other modifications in it.
So the reload is prevented by a mod? And not from thermos / cauldron?
That depends on it. If the modifications are in a package from Thermos / Claudron, then from Thermos / Claudron. Did you add the modifications yourself, then from the modification.
I've created the modpack and the server completely by myself but how do I find out which mod prevents the reload?
You're asking the wrong one. I only own a BungeeCord / Spigot server, it stays that way. I do not believe in mod-packs.
One should never reload, because it triggers errors.
You can change that in the configs. I would advise against it! Reloads are generally not good for servers. And if errors occur then you usually get even no support! Just restart the server. That's the way to do it.