Hi, 
I'm new here.
Got the same error here my config.
apache2:
LoadModule proxy_http_module /usr/lib/apache2/modules/mod_proxy_http.so
LoadModule proxy_module /usr/lib/apache2/modules/mod_proxy.so
                AllowEncodedSlashes On
                RewriteEngine on
                RewriteRule ^/calibre/(.*) http://127.0.0.1:81/calibre/$1 [proxy]
                #RedirectMatch permanent ^/calibre$ /calibre/ #I tried both, same error
                RedirectMatch 301 ^/calibre$ "/calibre/"
My Error message:
Traceback (most recent call last):
    at XMLHttpRequest.ρσ_anonfunc (
https://someurl.test/calibre/:4895:21)
    at complete_callback (
https://someurl.test/calibre/:4864:21)
    at set_error (
https://someurl.test/calibre/:4820:33)
Error: Failed to read the 'responseText' property from 'XMLHttpRequest': The value is only accessible if the object's 'responseType' is '' or 'text' (was 'blob').
Can somebody help me what I made wrong?
Thx a lot for your help