I did a quick google and found some similar reports (although not the same) but they were for much older versions of rsync. So I doubt they are related.
The fact that you said the exact same command works fine in v14.x suggests that there may be a regression in rsync. Although, having said that, I checked the Debian bug tracker and there doesn't appear to be anyone reporting a similar bug against rsync v3.1.2 (the version in Debian 9/Stretch / TurnKey v15.x). So that would suggest, that even if it is a regression, the circumstance must be pretty specific. Are there any other factors you think may be in play here? E.g. are they syncing to the same remote host? Are the same files being transferred? If not, perhaps try syncing across your 2 machines and see if that changes things. I.e. see if the problematic files are ok on v14.x and/or see if the ok files are a problem on v15.x.
I also suggest that you try increasing the verbosity of rsync and see if anything specific shows up. I.e. -v should give you more info than default, adding more v's should give even more info . I'm not sure how many v's it maxes out at, but the man page says this:
A single -v will give you information about what files are being transferred and a brief summary at the end. Two -v options will give you information on what files are being skipped and slightly more information at the end. More than two -v options should only be used if you are debugging rsync.
Regardless, it may be worth opening a bug against rsync on Debian. TurnKey is basically a customised Debian and we install rsync (plus most of the rest of the OS) directly from Debian repos. v14.x was based on Debian Jessie; v15.x is based on Debian Stretch. The rsync package maintainer(s) may have some further more specific guidance on troubleshooting and gathering further info.
If you've never reported a bug to Debian before, you might find it a bit bizarre (it's quite different to posting a bug on a web based bug tracker). Debian do provide a fair bit of documentation (e.g. see here, here and/or herre) but I just came across a video on YouTube which looks quite good (TBH I actually didn't watch it all - but it seemed pretty good).
Please post back on how your troubleshooting etc goes. I'll try to help out as much as I can.
I suggest that you try turning up verbosity.
I did a quick google and found some similar reports (although not the same) but they were for much older versions of rsync. So I doubt they are related.
The fact that you said the exact same command works fine in v14.x suggests that there may be a regression in rsync. Although, having said that, I checked the Debian bug tracker and there doesn't appear to be anyone reporting a similar bug against rsync v3.1.2 (the version in Debian 9/Stretch / TurnKey v15.x). So that would suggest, that even if it is a regression, the circumstance must be pretty specific. Are there any other factors you think may be in play here? E.g. are they syncing to the same remote host? Are the same files being transferred? If not, perhaps try syncing across your 2 machines and see if that changes things. I.e. see if the problematic files are ok on v14.x and/or see if the ok files are a problem on v15.x.
I also suggest that you try increasing the verbosity of rsync and see if anything specific shows up. I.e.
-vshould give you more info than default, adding more v's should give even more info . I'm not sure how many v's it maxes out at, but the man page says this:Regardless, it may be worth opening a bug against rsync on Debian. TurnKey is basically a customised Debian and we install rsync (plus most of the rest of the OS) directly from Debian repos. v14.x was based on Debian Jessie; v15.x is based on Debian Stretch. The rsync package maintainer(s) may have some further more specific guidance on troubleshooting and gathering further info.
If you've never reported a bug to Debian before, you might find it a bit bizarre (it's quite different to posting a bug on a web based bug tracker). Debian do provide a fair bit of documentation (e.g. see here, here and/or herre) but I just came across a video on YouTube which looks quite good (TBH I actually didn't watch it all - but it seemed pretty good).
Please post back on how your troubleshooting etc goes. I'll try to help out as much as I can.