Hi, I'm getting the same error - just when I click the Agile tab. I found it could be connected (but not necessary) to another plugins installed later (I installed WBS and Easy Gantt from EasyRedmine).
Another idea is that we both have Redmine 4.2.0 and the Agile plugin is ready for 4.1 as I found somewhere in specification.
My environment versions:
Environment:
Redmine version 4.2.0.stable
Ruby version 2.6.6-p146 (2020-03-31) [x64-mingw32]
Rails version 5.2.5
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp
SCM:
Git 2.31.1
Filesystem
Redmine plugins:
easy_gantt 1.12
easy_mindmup 1.0
easy_wbs 1.5
redmine_agile 1.6.0

Will leave a note if I have some success / news.

Hello. I would like to let you know that the reported problem about Agile plugin error on Redmine 4.2.0 is now fixed by our developers. Screenshot references:

1. Agile plugin fixed version installed on Redmine 4.2.0 ---> https://i.imgur.com/H5UAiYU.png
2. Agile board opened without error ---> https://i.imgur.com/Oa8gJL6.png

Please contact our support team at support@redmineup.com and mention what is the case about. Afterward our support engineers will send you the fixed version of the plugin. Thank you.

Dear Support team
I update Agile plugin to (Light) version 1.6.0 like you.
But it is also display that issue error 500 (log) below:
Completed 500 Internal Server Error in 34ms (ActiveRecord: 15.5ms)

NoMethodError (undefined method `groupable' for #<QueryColumn:0x000000000a094d18>
Did you mean? groupable=
groupable?):

plugins/redmine_agile/app/models/agile_query.rb:349:in `block in groupable_columns'
plugins/redmine_agile/app/models/agile_query.rb:349:in `select'
plugins/redmine_agile/app/models/agile_query.rb:349:in `groupable_columns'
app/models/query.rb:907:in `group_by_column'
app/models/query.rb:890:in `group_by_sort_order'
plugins/redmine_agile/app/models/agile_query.rb:493:in `issues'
plugins/redmine_agile/app/controllers/agile_boards_controller.rb:60:in `index'
lib/redmine/sudo_mode.rb:61:in `sudo_mode'.

My Redmine system infor:

Environment:
Redmine version 4.2.0.stable
Ruby version 2.6.6-p146 (2020-03-31) [x64-mingw32]
Rails version 5.2.5
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp
SCM:
Subversion 1.14.1
Git 2.31.1
Filesystem
Redmine plugins:
easy_mindmup 1.0
easy_wbs 1.5
redmine_agile 1.6.0
redmine_resources 1.0.4

I resolved this issue uninstalling others plugins. In my case i had redhopper plugin installed, after uninstall it agileboard works great