File can't be saved, not found - but is there

When we have several files that are saved on a Synology NAS, that is openend, worked in. After different time periods when saving we get error that file dos not exist. But the connection to server is not lot / disconnected. If we lookup the location we can save it, and overwrite the existing file. No time frame can e found when it happens. No sleep of the machine, using SU2023.