First of all, many thanks to @fancy__04 for reporting this issue.

So basically in ‘Manual Mode’ there is a bug when attempting to redefine a custom payload in case the first attempt failed, for any reason that might be.
A simple example of when that can be caused, is by mistyping the filename of the custom payload.
The bug relies in the fact that the buffer holding the previous name is not re-initialized. This means that if the next filename is shorter than the previous one, then the extra characters of the previous name will remain ‘appended’ in the current one.
This basically would lead again to an invalid name until another valid filename longer or equal to the first one is set. This does not apply in ‘Auto Mode’ as there the buffer is correctly re-initialized when needed.
A fix for this issue will be applied in the upcoming Shellter v6.8.

Cheers,
kyREcon