After installing some packages on my Debian VPS, I couldn't reboot my webservice (lighttpd). This error kept appearing: 

2018-04-12 19:34:00: (server.c.1201) couldn't get 'max filedescriptors' Function not implemented
After searching the interwebz I could not find a proper fix. A solution seemed nowhere to be found, so I just downloaded lighttpd and statically set the max filedescriptor like this in server.c: 
srv->max_fds = 1024; //rlim.rlim_cur;
Just comment out all the surrounding code, and it works! I hope this helps others.

For clearance, here is the source code I commented out: 

1189                  if (0 != getrlimit(RLIMIT_NOFILE, &rlim)) {
1190                          log_error_write(srv, __FILE__, __LINE__,
1191                                          "ss", "couldn't get 'max filedescriptors'",
1192                                          strerror(errno));
1193                          return -1;
1194                  }
1195  
1196                  /**
1197                   * if we are not root can can't increase the fd-limit above rlim_max, but we can reduce it
1198                   */
1199                  if (use_rlimit && srv->srvconf.max_fds
1200                      && (i_am_root || srv->srvconf.max_fds <= rlim.rlim_max)) {
1201                          /* set rlimits */
1202  
1203                          rlim.rlim_cur = srv->srvconf.max_fds;
1204                          if (i_am_root) rlim.rlim_max = srv->srvconf.max_fds;
1205  
1206                          if (0 != setrlimit(RLIMIT_NOFILE, &rlim)) {
1207                                  log_error_write(srv, __FILE__, __LINE__,
1208                                                  "ss", "couldn't set 'max filedescriptors'",
1209                                                  strerror(errno));
1210                                  return -1;
1211                          }
1212                  }
1213  
1214                          srv->max_fds = rlim.rlim_cur;
1215                          /*(default upper limit of 4k if server.max-fds not specified)*/
1216                          if (i_am_root && 0 == srv->srvconf.max_fds && rlim.rlim_cur > 4096)
1217                                  srv->max_fds = 4096;
1218  
1219                  /* set core file rlimit, if enable_cores is set */
1220                  if (use_rlimit && srv->srvconf.enable_cores && getrlimit(RLIMIT_CORE, &rlim) == 0) {
1221                          rlim.rlim_cur = rlim.rlim_max;
1222                          setrlimit(RLIMIT_CORE, &rlim);
1223                  }