First:
Apple contacted me regarding my question. The iWork team asked me to share system diagnostics and how to reproduce the issue. Hopefully this might help.
Second:
I found a workaround which helped me out of this situation.
Long story short:
Switch to SMB. It is the new standard of macOS.
Long story long:
I had trouble with just launching SMB because a lot of directories and files were named with special characters like a slash. And I had a lot of folders with a space at the end or a period at the end of the name. This caused SMB to show these files and directories with so called mangled names. (There are workarounds to disable name mangling, but this caused my affected folders to be empty.) The files were named via AFP, which didn't cause any issues at all so far.
Of course it is impossible to seek and change the Terrabytes of files and directories manually.
I reseachred like 3 entire days, tested a lot and finally learned a lot:
I created a bash script which lists all the files and directories with bad names. I ran it via SSH. The list is formatted and saved to a .csv file which allowed me as a human to catch the paths and names easily to get an overview of the situation.
After that I changed the script, that it actually created another bash script which sequentially replaced the characters in the file and directory names with underscores and added a random number at the end to prevent overwriting data.
I created a .csv file again to check whether my plan worked – with an empty result.
I now have AFP and SMB running.
AFP is used for Time Machine backups, there is one single share on the server for time machine backups only. Besides the admin accounts, only one time machine user has access to that share. The login data for that time machine user is saved on all Macs keychains. This is working perfectly.
All other network shares are accessed via SMB, the macOS default. I can just click the server in the Finder's sidebar. Login data are stored in keychain as well, but obviously only for smb://server.local.
Make sure to delete every login details related to the server from keychain to ensure that Finder is only connecting via SMB.
That's it. Numbers, Word, TextEdit, Preview and the others are working perfectly again via SMB without the problem of crashing Finder while the process of saving a file.
The irony:
Connected via SMB, files and folders named with the same special characters which drove me crazy days ago, are not shown mangled. No Problems at all now. (I guess its because smb uses another character coding ..., what ever)
I hope this helped anyone. If you are facing the same problems just ask me. I can describe the procedure more detailed.