No, The problem was not in webdav or notes. It's not even directly related to joplin.
Luckily I was able to find the solution.
I search the error on google as "unexpected char 0x131 at 0 in".
I saw a github link on google. Clicked it. There was a discussion where it was said that the upload feature of an application called Rocket chat did not work in Turkish.
I changed the language of my phone to English. (Not the language of Joplin, but the language of the phone.)And once again I synced. And I got no errors. After synchronizing once, I no longer have a synchronization problem, even though I change the language to Turkish as before. To make sure of that, I deleted and reinstalled the app. Again it only worked in english. After the first synchronization is completed, there is no problem if I change the language to Turkish.
Here is github discussion:
https://github.com/RocketChat/Rocket.Chat.ReactNative/issues/3040#issuecomment-865032744
Here is another user has same problem:
https://github.com/joltup/rn-fetch-blob/issues/573#issuecomment-1021001462
I've also seen users having other sync problems in google play comments in turkish language. I wrote the solution in google play comments.
Thanks for the replies
