The error is easily explained. You're expecting readtable to read the first column of your csv as a datetime. You haven't checked that it does and it turns out that it doesn't (because it's not obvious to matlab that it is a date). It reads the column as text so date0 ends up as a cell array.
One way to force matlab to read the column as date:
opts = detectImportOptions('DTB3.csv');
opts = opts.setvaropts('DATE', 'Type', 'datetime', 'InputFormat', 'dd.MM.yy', 'DatetimeFormat', 'dd MMM yyyy');
RiskFree = readtable('DTB3.csv', opts);
Note that the way the date is encoded in your file is ambiguous. If you can, change whatever creates these files so that it encodes the year with 4 digits.