Biber 2.14 - Puzzling repeated error : "Invalid format '2020-01-01' of date field 'date' - ignoring"

Biber is written in Perl, but to make it usable for people without a Perl installation and to make sure that all dependencies have the right versions, it is distributed in 'compiled' binary form that bundles all Perl dependencies using PAR::Packer.

On the first Biber run, Biber will unpack all its dependencies into a temporary cache location. Subsequent runs will use the dependencies from the cache.

Sometimes the cache gets corrupted during unpacking or afterwards (maybe due to anti-virus software). In that case Biber may behave quite weirdly for no apparent reason, sometimes you get useful errors, sometimes not.

In the past there was a very common error about a missing recode.xml: Biblatex/biber fails with a strange error about missing recode_data.xml file.

In cases of weird Biber errors, delete the Biber cache as described in Biblatex/biber fails with a strange error about missing recode_data.xml file. (If Biber is still working well enough biber --cache should give you the cache location. Otherwise you have to find the location manually, that is described in the linked answer as well as the Biber documentation)

Then rerun Biber (for example with biber --help from the command line), it is important that this Biber run can finish cleanly without interruption. Do not kill the process even if it takes way longer than you expected. (Since Biber unpacks itself this run takes longer than a usual Biber run.)

Recent issues with the Biber cache are https://github.com/plk/biber/issues/312 and quite probably also Biber 2.14 broken ? - Biber warning: [557] Utils.pm:304> WARN - Entry ...' (....bib): Invalid format '2018' of date field 'date' - ignoring.

As moewe guessed in his comment, the issue was Biber's cache being corrupted, and deleting the cache fixed it.