Comment 12 for bug 1842701

can you try something like this since you had a sandbox env:

Index: modules/proxy/mod_proxy_balancer.c
===================================================================
--- modules/proxy/mod_proxy_balancer.c (revision 1866509)
+++ modules/proxy/mod_proxy_balancer.c (working copy)
@@ -1185,7 +1185,7 @@
     /* Ignore parameters if this looks like XSRF */
     ref = apr_table_get(r->headers_in, "Referer");
     if (apr_table_elts(params)
- && (!ref || !safe_referer(r, ref))) {
+ && (ref && !safe_referer(r, ref))) {
         ap_log_rerror(APLOG_MARK, APLOG_ERR, 0, r, APLOGNO(10187)
                       "ignoring params in balancer-manager cross-site access");
         apr_table_clear(params);

No referer should pass through IIUC.