This is the cause of issue 7021 that was closed.

This is run under Windows 10

JabRef 5.2--2020-10-19--af203b6
Windows 10 10.0 amd64
Java 14.0.2

Extra character is automatically added by Jabref to bib field "file" when an arXiv article was added to bib file using the ID "arXiv" option in "New entry". This caused errors when running "Check integrity" (Quality -> Check Integrity). Also, sometimes it causes errors when saving the bib file in JabRef, and thus I need to manually remove it when adding an arXiv article

Minimally reproducible example:

1. Add the arXiv article https://arxiv.org/abs/2010.08497v1 to a bib file
2. The automatically created content for field "file" is file = {:http://arxiv.org/pdf/2010.08497v1:PDF},