I confirm - for what it’s worth - that by enabling the “Ignore permissions” function (nested in Actions (top right)> Settings> General> Default configuration> click on> Change folder defaults> Advanced> flag on “Ignore permissions” (here) the synchronization proceeds and completes successfully between different OSes: Windows android and DSM7 (or later) Synology. Setting not very easy to find, but effective and working. Synchronizations that were not initially completed have all been successful.
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Those who read and were able to intervene in the Syncthing update, could consider inserting a switch in Actions> Settings> Ignored folders - to delete - intentionally - folders created by mistake and deleted and / or replaced in the device / s. I think it is useless, especially after some time, to keep the evidence of objects that no longer exist, perhaps creating doubts or confusion