The errors suggest the component load order is breaking, not the files themselves. If OssnMessages isn’t being detected, OssnChat will fail every time since it depends on it. I’d check whether the component folder names and ossn_com.xml files still match exactly geometry dash lite, because a small mismatch can cause OSSN to skip loading a component even if the PHP files are present. Have you already tried clearing the OSSN cache and regenerating it?

OMG there was something indeed in the cache that was blocking the site!!!!
Thanks a lot man! Where is your buy me a coffee link!?

Ok,
most likely you applied code changes with Ossn cache still enabled, you did a mistake and that mistake gets cached.
So step 1 is to asure that cache is off in the database
Start PhpMyAdmin, enter the ossn_site_settings table and make sure the cache value looks like in my screenshot

If not, then change it from 1 (one) to 0 (zero)

Step 2 is to disable the component (DisableMemberSelfValidating) that most likely was causing the initial crash
So enter the ossn_components table, find the corresponding row and set it to 0 the same way (as i did in my example here)

If you are not sure if other components may be involved too, then disable them the same way...

Now try to call your site again. If it's still crashing, check and provide the error_log again

Ow ok I understand.
So 2 days ago, it suddenly gave a white page when I was editing texte displayed into another component.
Step 1: I reverted my changes in this plugin but still white page.
Step 2: troubleshooting by disabling one by one the components and then I realized that by renaming OssnMessages to -OssnMessages, white page changed to an OSSN page with the error code. Ok.
Step 3: I looked into the changes performed in the php pages and nothing found
Step 4; so what I did is reupload a full backup of pages to my sftp from the date I had no issue bit didn't resolve
Step 5: I restored a 3 days le DB and even with this, site is still down

So here I am.

Franck,
you sent us that error_log
and you wrote you renamed OssnMessages and OssnChat

I was just trying to understand,
so I did it as you did, and voila: got the same error - my site was down and of course I had NO access to my admin panel any more.

Okay, the reason for that error is explained now.

So you need to undo the renaming (make sure that OssnMessages and OssnChat are back in place
as any other file you may have renamed.

But I'm still confused because you wrote: "I reuploaded a backup of all php files from when it worked 3 weeks ago: same issue."

So actually, OssnMessages and OssnChat should be back in place already. Or did you do that renaming after uploading your backup?
Next question: Did you restore the PHP files only, or did you restore a backup of your database, too?

Thanks for the input Michael.
Is your OSNS also down or do you also get the error?

on renaming/deleting OssnMessages while this component is still marked as ENABLED on my administrator/components page.

Do you mean you have an access to your admin panel, even with this error(s)? I DON'T even have :-(

Please explain what you're trying to achieve.

I have a doubt: you talk to me?
Because, when this happened, I was editing the DisableMemberSelfValidating component and then something made that the website went all white. And the modification I made was about text displayed after creating an account.

This makes me think: is there somewhere a file where it's coded if a component is activated or disactivated? Maybe I can try to fix this way?

And, nobody from OSSN team available for any help?

Thx-

I can confirm getting exactly the same issue

Error: Class "OssnMessages" not found in /components/OssnChat/classes/OssnChat.php:11

on renaming/deleting OssnMessages while this component is still marked as ENABLED on my administrator/components page.

But why the hell would I do that?

ALL files of an Ossn component are expected to be in place as long as this component has been enabled from your adminstrator->components page. Some components even rely on other components to be in place and enabled.
Please explain what you're trying to achieve.