Bidirectional Syncing with Android using Unison and MTP

I'm currently trying to sync bidirectionally some folders of my PC (Ubuntu 18.04) with my Smartphone (Fairphone 3, Android 9) via MTP using Unison. I thought that this should be pretty easy, but it turned out to be rather complicated. Unison is able to detect if two versions of a file exist in the two sync directories. However, than two problems occur:

1. Unison cannot detect which one of the files is the up-to-date version. I can set this manually, but this is rather time consuming and also prone to errors, in case I make a mistake.
2. Unison cannot overwrite the old version of the file with the up-to-date version.

I here document what I have done so far:

1. Create a mount directory for the file system of the smartphone:

mkdir /tmp/fp3

2. Mount the filesystem of the phone:

jmtpfs -o allow_other /tmp/fp3

3. Sync the directory of the phone with the according directory of the PC. Both directories contain different versions of the file "test":

unison -fat ~/Desktop/Test /tmp/fp3/SD-Card/Home/Test

The last command returns the following output in the console. Unison cannot detect the appropriate syncing direction for the file "test" and I'm setting it manually with ">":

Contacting server...
Looking for changes
Reconciling changes
Desktop/Test Home/Test
new file ====> new file test [] >
Proceed with propagating updates? [] y
Propagating updates
UNISON 2.48.4 started propagating changes at 10:29:48.42 on 06 Mar 2020
[BGN] Updating file test from /home/$USER/Desktop/Test to /tmp/fp3/SD-Card/Home/Test
Failed: Error in renaming /tmp/fp3/SD-Card/Home/Test/.unison.test.92833400e4472a77b19cca039512131e.unison.tmp to /tmp/fp3/SD-Card/Home/Test/test:
Input/output error [rename(/tmp/fp3/SD-Card/Home/Test/.unison.test.92833400e4472a77b19cca039512131e.unison.tmp)]
100% 00:00 ETAFailed [test]: Error in renaming /tmp/fp3/SD-Card/Home/Test/.unison.test.92833400e4472a77b19cca039512131e.unison.tmp to /tmp/fp3/SD-Card/Home/Test/test:
Input/output error [rename(/tmp/fp3/SD-Card/Home/Test/.unison.test.92833400e4472a77b19cca039512131e.unison.tmp)]
UNISON 2.48.4 finished propagating changes at 10:29:48.99 on 06 Mar 2020
Saving synchronizer state
Synchronization incomplete at 10:29:48 (0 items transferred, 0 skipped, 1 failed) failed: test

Looks like Unison is first copying an up-to-date version of the file "test" with the name ".unison.test.92833400e4472a77b19cca039512131e.uni son.tmp" into the directory where the old version of "test" sits. Then it is trying to rename the up-to-date version, which seems to fail. Any ideas what could cause this error? Any help is highly appreciated!

Cheers

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