The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The “License” field contains a short name with a bad exception format. According to specification format of exception is: shortlicencename with exceptionname exception.
If more than one exception applies to a single license, an arbitrary short name must be used instead.
Please refer to https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ for details.