Thanks for the response :). After spending several days on this issue, I’ve come up to the conclusion.
The very first severe problem was, I was throwing the exception from the localhost 
. So in order to debug the issue I created a small react project composed from one minified .js file with a sourcemaps reference and one generated source map file. Then deployed the project to now.sh, threw an exception a it worked. (No source maps uploaded yet).
Similarly it worked when we generated sourcemaps for our internal nextjs project, deployed and it worked. (Still no source maps uploaded yet).
Yesterday I realised I hadn’t uploaded sourcemaps at all yet. So took the small react project once again, uploaded the sourcemaps, generated a minified .js file without a sourcemaps reference and deployed to now.sh. After throwing an exception everything worked but at this moment the sourcemaps were mapped onto the minified js file…which is not that bad at all because I could see where the exception was thrown but you know…it’s still a minified file 
Regarding the internal nextjs project and uploaded source maps - haven’t given a try yet. But I’m a bit sceptic it won’t work correctly because of the issue you described above. Will give a try tomorrow.
The conclusion - I’d better not to upload the sourcemaps to Sentry for a nextjs project if not necessary and simply use sourcemaps references - //# sourceMappingURL=show.js.map - created during a compilation/transpilation process.