The link given by TheBigLebowski also refers to:
Disable multiple connections from the same IP address: When this option is enabled, the Synology NAS only allows one connection per IP address from clients using the SMB protocol. When multiple clients with same IP address use the SMB protocol, the Synology NAS will keep the latest one and drop any older connections. In a NAT environment, the client might be using a single IP address, in which case enabling this option might affect the normal performance.
This in not ticked on my NAS, but might just be worth checking? Control Panel, File Services, SMB, Advanced.
Disable multiple connections from the same IP address: When this option is enabled, the Synology NAS only allows one connection per IP address from clients using the SMB protocol. When multiple clients with same IP address use the SMB protocol, the Synology NAS will keep the latest one and drop any older connections. In a NAT environment, the client might be using a single IP address, in which case enabling this option might affect the normal performance.
This in not ticked on my NAS, but might just be worth checking? Control Panel, File Services, SMB, Advanced.