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I had this same problem and looked at the difference between your mods and mine and found that the added origins mod is the culprit of this issue, though I do not know how. This helped me to solve my issue. The server should be able to load, if you don't have any other bugs, if you delete that mod and replace it with the version before it:
replace
Added Origins V3.1.0
with
Added Origins Halloween 3.0.0-B (fixed) 1.16.5 - 1.17.1
Thankyou for the quick response, and I do understand that customizing modpacks might not be possible due to technical reasons. It seems that I made this post prematurely without exploring properly as I have found that going to files you can change the server.properties, though it would be nice if the options tab defaulted to this page as they are different (see attached pictures for example.)
Sad that there is nothing to be done about the world generation. Maybe there is a way if you change the level.dat?
Again, thankyou for responding to my post and I apologize for not doing proper research before making a post.
I recently created a server for me and a friend on aternos, using the Valhelsia 3 modpack software, which has Biomes O' Plenty as one of the mods. I generated a world, without realizing that the default generation (Set by level-type in server.properties file) was vanilla generation and not Biomes O' Plenty generation. We played for a while without noticing that we were missing some biomes, and when we did, we already had progress in the world. With the current system in Aternos, the level-type can only be changed before world generation. Therefore we could not explore the biomes from the mod without restarting our progress.
Had we been able to change this property post-generation, then future world generation in yet unloaded chunks would have, to the best of my knowledge, been changed to that of Biomes O' Plenty generation, while our current progress in the world and explored chunks would be the same.
What I am proposing to Aternos is to give users more freedom over server files in general, so that these properties could be changed at any point. This could also be implemented for various other files in Aternos to allow users more freedom over their servers in general, such as the mod folder, so that you could customize pre-installed modpacks and add or delete mods based on how the user wants their experience.
However, the most pressing problem for me at the moment is the lack of options with the server.properties file, not only for world generation but other things such as view-distance (Though this might have an impact on performance, since Aternos has a Hardset limit to how much RAM the server can use, this would not impact Aternos and would allow users to sacrifice some server speed or the amount of mods able to run for better graphics ) and spawn-animals/monsters/npcs.
At the very least, if none of this could be implemented, I would at least like the option to change the level-type post world generation within the options tab in Aternos.
Thank You for reading,
Argitechno
