I think there are two errors, but maybe I'm wrong.
One is that if you modify the name of a plugin in 'b2evo > System ? > Plugin configuration' (I tried with Archives Widget, both in local and in demo3.b2evolution.net), then the plugin wouldn't work, because internally the plugin code is 'evo_Arch', and nothing else.
The second is that if you try to go back to the original name, you get an error: 'The plugin code is already in use by another plugin.', even if you can see in the plugins table that this is not true!
I'm not sure if I've done something wrong, but it seems that when converting from old b2evo versions to newer, the system puts the value of $tableprefix in front of the code of the plugin. So, normally will be 'evo_Arch', but if you change tableprefix to xxevo_, the plugin's code appears as 'xxevo_Arch'
Can you please take a look at it?
@fede thanks for reporting, the both of them look like bugs indeed. The dev team will evaluate and determine that and, as usual, the fixes will be available in a next release.
I can't find such a hint about that in the upgrade code. Do you remember from what version did you upgrade?