I've found the problem! I'll post bug report soon -
Saving as "Project folder" is broken in the latest beta.
Go into C3, open Kiwi Story, save it as project folder, and upon completion, you get the same error as me (and it persists after refreshing C3).
The really strange thing is, when I opened the new beta earlier today for the first time, I didn't even open my project yet, and the error appeared at the top of C3. Maybe C3 doesn't like the "Recent Items" history if it has a project folder? Who knows.
Since it pops up, I can copy the whole error from triggering this in Kiwi Story:
Error report information
Type: unhandled rejection
Reason: Error: missing placeholder '{0}' in string substitution Error: missing placeholder '{0}' in string substitution at Function.ip.fTa (https://editor.construct.net/r278/main.js:1008:245) at q8b (https://editor.construct.net/r278/main.js:1027:477) at Function.f.q0 (https://editor.construct.net/r278/main.js:200:274) at window.aic.Idb (https://editor.construct.net/r278/components/misc/startPage2/startPage2.js:9:342) at xib (https://editor.construct.net/r278/components/misc/startPage2/startPage2.js:4:177)
Stack: Error: missing placeholder '{0}' in string substitution at Function.ip.fTa (https://editor.construct.net/r278/main.js:1008:245) at q8b (https://editor.construct.net/r278/main.js:1027:477) at Function.f.q0 (https://editor.construct.net/r278/main.js:200:274) at window.aic.Idb (https://editor.construct.net/r278/components/misc/startPage2/startPage2.js:9:342) at xib (https://editor.construct.net/r278/components/misc/startPage2/startPage2.js:4:177)
Construct 3 version: r278
URL: editor.construct.net/r278
Date: Tue Jan 11 2022 16:47:11 GMT+0000 (Greenwich Mean Time)
Uptime: 30.1 s
Platform information
Browser: Chrome
Browser version: 97.0.4692.71
Browser engine: Chromium
Context: browser
Operating system: Windows
Operating system version: 10
Device type: desktop
Device pixel ratio: 1
Logical CPU cores: 12
Approx. device memory: 8 GB
User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/97.0.4692.71 Safari/537.36
C3 release: r278 (beta)
Language setting: en-US
WebGL information
Version string: WebGL 2.0 (OpenGL ES 3.0 Chromium)
Numeric version: 2
Supports NPOT textures: yes
Supports GPU profiling: yes
Supports highp precision: yes
Vendor: Google Inc. (NVIDIA)
Renderer: ANGLE (NVIDIA, NVIDIA GeForce RTX 2060 Direct3D11 vs_5_0 ps_5_0, D3D11-30.0.14.9729)
Major performance caveat: no
Maximum texture size: 16384
Point size range: 1 to 1024
Extensions: EXT_color_buffer_float, EXT_color_buffer_half_float, EXT_disjoint_timer_query_webgl2, EXT_float_blend, EXT_texture_compression_bptc, EXT_texture_compression_rgtc, EXT_texture_filter_anisotropic, EXT_texture_norm16, KHR_parallel_shader_compile, OES_texture_float_linear, WEBGL_compressed_texture_s3tc, WEBGL_compressed_texture_s3tc_srgb, WEBGL_debug_renderer_info, WEBGL_debug_shaders, WEBGL_lose_context, WEBGL_multi_draw, OVR_multiview2