A couple of updates:
Another example where repeatedly clicking Render fails to set Clients to Working. Stop and relaunch Server, then start Render again works fine.
I find it of interest because the same settings, except with the camera at a greater distance, haven't caused so much problem. And, the closer I move in the worse it seems to get.
# Mandelbulber settings file
# version 2.11
# only modified parameters
[main_parameters]
aux_light_colour_1 ff00 ff00 ff00;
aux_light_enabled_1 true;
aux_light_intensity_1 4.1;
aux_light_position_1 4 -4 4;
background_color_1 0000 0000 0000;
background_color_2 2a00 3e00 3600;
background_color_3 ff00 ed00 ab00;
boolean_operator_1 0;
boolean_operator_2 0;
boolean_operator_3 0;
boolean_operators true;
box_folding_limit 1.5;
camera -0.08733398805806962 -0.08292766540109509 -0.08139408355882762;
camera_distance_to_target 0.1198842972517017;
camera_rotation 137.9749721735613 -31.95097150828001 -8.000000000000011;
camera_top -0.2474384205152353 -0.4824570194695414 0.8402436863300388;
DE_factor 0.1;
delta_DE_function 2;
detail_level 2;
flight_last_to_render 0;
formula_1 88;
formula_2 83;
formula_3 7;
formula_material_id_1 2;
formula_material_id_2 3;
image_height 272;
image_width 480;
julia_c 2.774110549353855 -2.656528596132875 1.904831177860441;
julia_mode_1 true;
keyframe_last_to_render 0;
main_light_alpha 24;
mat1_is_defined true;
mat1_surface_color_palette fd6029 698403 fff59c f5bd22 0b5e87 6a533f 2282a5 3b9fee d4ffd4 aba53c;
mat2_is_defined true;
mat2_shading 0.8;
mat2_surface_color_palette fd6029 698403 fff59c f5bd22 0b5e87 c68876 64103d 3ba4ef d4ffd4 aba21b;
mat2_transparency_of_surface 0.5;
mat3_is_defined true;
mat3_surface_color_palette fd6029 7b7402 fff59c f5bd22 633100 301b15 aa0000 f87c00 d4ffd4 f1e754;
mat4_fresnel_reflectance true;
mat4_is_defined true;
mat4_name glass;
mat4_reflectance 1;
mat4_specular 20;
mat4_specular_width 0.108;
mat4_transparency_interior_color 1900 3f00 0000;
mat4_transparency_of_interior 0.74131;
mat4_transparency_of_surface 1;
mat4_use_colors_from_palette false;
raytraced_reflections true;
spherical_folding true;
target -0.1554322893465567 -0.1584920855007935 -0.1448360610197572;
view_distance_max 36.06895516380957;
[fractal_1]
IFS_rotation_enabled true;
[fractal_2]
IFS_rotation_enabled true;
[fractal_3]
IFS_rotation_enabled true;
[fractal_4]
IFS_rotation_enabled true;
Only once have I seen Clients start working while Server already Rendering. I mean, normally they don't start until rendering is started again after connecting.
In this case they started working immediately upon connection. But, both clients produced the same 'error' as illustrated in the attachment.