it seems Power BI Desktop ( Version: 2.69.5467.2151 64-bit (May 2019) )
corrupts files at saving.
i just made the experiment of opening a copy of a working report, click save and closed the file just to get an error trying to open it again.
My reports only use one data source, a direct query sql data source, on the sql server side i can see all the failed login attepts for user '' (blank), at the same momen i have tryied to open the damaged files, so the data source information works except for the credentials that seems not to be saved correctly.
I get the same errors if i download a pbix copy of a published report, i cant get it to open the file, nor i get prompted to enter valid credentials nor it opens the report file so i can use the query edition tools to fix the querys.