A family of Microsoft relational database management systems designed for ease of use.
I would make the registry change on each Windows 10 client that uses the Access files that are on the SMB share (then restart the PCs). I don't know anything about Synology, but if you make the above change on the Windows 10 clients, it will probably resolve the problem. If Synology has the equivalent of Disable Leasing, you could try that too or instead.