Issues refreshing an access datasource

We are facing a very strange issue on scheduling a cache refresh on a datasource to an access database.

the refresh fails with these  errors (they are randomly alternating)

[0] -1055784932: Unspecified error: E_FAIL(0x80004005). The exception 
was raised by the IDbCommand interface.

2020-01-30 11:40:09.9865|ERROR|116|Error Processing Data Model Refresh: SessionId: 2c213e22-0799-45e0-a89b-38623fb9d385, Status: Error Failed Data Refresh, Exception Microsoft.AnalysisServices.OperationException: Failed to save modifications to the server. Error returned: 'Credentials are required to connect to the File source. 
(Source at \\xxxxftp\dnd\xx\xxx events\filiali\xfiles.2019.accdb.). The exception was raised by the IDbCommand interface.

if we connect to the server and launch a process monitor to see what is going behind the scene, the refresh goes fine without any error...

we cannot figure out if and what privilege is missing or if is an issue of other kind (the Antivirus- which we cannot disable- is showing nothing)

thanks

Power BI version September2019

windows OS: windows sever 2016 standard

database server :Microsoft SQL Server 2017 (RTM-CU15-GDR) (KB4505225) - 14.0.3192.2 (X64)
Jun 15 2019 00:45:05
Copyright (C) 2017 Microsoft Corporation Enterprise Edition: Core-based Licensing (64-bit) on Windows Server 2016 Standard 10.0 <X64> (Build 14393: ) (Hypervisor)