I have compared the two logs and the conclusion is:
Rebex client sends always same commands for FileExists() and Rename(), but the server replies differently.
The only meaniangful explanation is that the state on the server was different (in old log, the file existed, but it doesn't exist when creating new log).
In old log you can see that for FileExists(), the server returned size for the file:
2019-03-21 15:13:02.190 INFO Ftp(1) Command: SIZE /b5a58610-8168-46a0-bb2f-d8a2b6be480f.mov
2019-03-21 15:13:02.443 INFO Ftp(1) Response: 213 112357617
While in the new log, the server returned no such file:
2019-03-19 17:06:13.970 INFO Ftp(1) Command: SIZE /b5a58610-8168-46a0-bb2f-d8a2b6be480f.mov
2019-03-19 17:06:14.222 INFO Ftp(1) Response: 550 /b5a58610-8168-46a0-bb2f-d8a2b6be480f.mov: No such file or directory.
For the rename, it is similar. Old log shows that the file is not on the server:
2019-03-21 15:15:01.569 INFO Ftp(1) Command: SIZE //New Name.mov
2019-03-21 15:15:01.822 INFO Ftp(1) Response: 550 //New Name.mov: No such file or directory.
But in the new log, it shows that the target file is there:
2019-03-19 17:06:36.769 INFO Ftp(1) Command: SIZE //New Name.mov
2019-03-19 17:06:37.019 INFO Ftp(1) Response: 213 110908443
I think, that the code will work with newer version as well.
But at first, please ensure manually (e.g. using FileZilla) that the file "/b5...0f.mov" exists on the server and the file "/New Name.mov" does not exists on the server.