I've attached the zipped file in case the invalid character gets lost in the description.
$ ruby -v
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin20]
$ solargraph -v
0.44.1
$ ls
test.rb
$ cat test.rb
#� Foo
bar = 1
$ solargraph scan
/Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source.rb:363:in `gsub': invalid byte sequence in UTF-8 (ArgumentError)
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source.rb:363:in `block in stringify_comment_array'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source.rb:361:in `each'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source.rb:361:in `stringify_comment_array'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source.rb:258:in `comments_for'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/parser/node_processor/base.rb:58:in `comments_for'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/parser/rubyvm/node_processors/lvasgn_node.rb:18:in `process'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/parser/node_processor.rb:38:in `process'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/parser/rubyvm/class_methods.rb:29:in `map'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source_map/mapper.rb:26:in `map'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source_map/mapper.rb:50:in `map'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/source_map.rb:157:in `map'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/library.rb:455:in `block in map!'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/library.rb:454:in `each'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/library.rb:454:in `map!'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/api_map.rb:119:in `load'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/shell.rb:177:in `block in scan'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/3.0.0/benchmark.rb:293:in `measure'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/lib/solargraph/shell.rb:176:in `scan'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/thor-1.1.0/lib/thor/command.rb:27:in `run'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/thor-1.1.0/lib/thor/invocation.rb:127:in `invoke_command'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/thor-1.1.0/lib/thor.rb:392:in `dispatch'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/thor-1.1.0/lib/thor/base.rb:485:in `start'
from /Users/halil/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/solargraph-0.44.1/bin/solargraph:5:in `<top (required)>'
from /Users/halil/.rbenv/versions/3.0.2/bin/solargraph:25:in `load'
from /Users/halil/.rbenv/versions/3.0.2/bin/solargraph:25:in `<main>'
I've attached the zipped file in case the invalid character gets lost in the description.