Your comments

What I am trying to say: 

Assuming you are talking about the mobile Nextcloud client app, which does not have syncing functionality(!) it only uploads files in 1 direction, that's it (called auto-upload): 

You were uploading a folder from A to B.
You then removed the application during uploading (never do such a thing again!).

This means B now has an incomplete folder and probably incomplete files.

But your files and folder on A are still OK. Can you confirm?

If you are talking about the Desktop client, that does do 2-way sync. I cannot imagine you removed that application while it was syncing!? But if you did and there were new/modified files on both A and B, at least the files that were syncing to B, should still be intact on A and vice versa.

FileRun or NextCloud client doesn't modify the file or folder that it syncs. Just copy/sync again. The original file or folder should still be on your device. Unless you deleted it.

@Vlad is it possible to share whether the OnlyOffice connector will be fixed/updated in the future?
It is not possible to edit new/existing XLSX files, you will always end up with blank, empty files.

Libvips seems amazing, does this mean it will become possible to perform the currently existing image editing functions without the metadata being destroyed? Will lossless rotation of images be possible)?
And tagging images, will it become possible to write the tag to the metadata of the file or to an XMP file?

Sorry for the questions, I am just excited :)

I can now confidently confirm this is a general issue, not unique to my installation.

I did a brand new install, without any persistent volumes, no database nothing.

I did docker-compose up -d for Filerun, mariadb and Onlyoffice.

Then I only configured OnlyOffice in FileRun Settings (the URL and JWT secret).

Same issue, only with XLSX files.

Please update the OnlyOffice connector. I fully understand focus shifted to support LibreOffice and that is also a great Office Suite, but it runs server side and it's UI needs some getting used to (which old people can't do anymore).

OnlyOffice is the onlye selfhosted Office Suite that runs client-side, relieving the server resources. 

I deleted my persistent /html folder (not the db folder and not the db container), my docker container and image and ran compose.

A new image was pulled, new container created, I used my license to activate.

I also removed my Onlyoffice container, image and persistent volume. 

I keep getting this issue after editing new or existing XLSX files (not with docx):

Thanks, then the issue must be with me. I'll figure it out or start with a new container and empty my html persistent volume. 

I think my first issue is, I don't know where to find the php.ini file that is mentioned here:

Other plugins I use do not use cURL/API (not sure about so you could still be right about the ssl issue. But the documentation does not state where to find or create the php.ini file.

I have created a new archive with the favicon assets for all browsers, operating systems and devices. I had cut off a bit too much originally.

Extract this to your /var/www/html folder:favicon.tar.xz and follow instructions above.

(The original highest quality asset in PNG and XCF (gimp) format:FileRun-logo-HQ.tar.xz)