There is a possibility, that this is a full system issue, cause Arc archive manager have troubles to display Czech chars as well and Kactivities are failing when try to extract to a subdir with czech special characters.
Since it was a fresh install of Manjaro KDE plasma, many users will experience this issue.
There is a strong chance this is an KDE bug, not an insync bug.
I am dumping my system information just in case
System: Host: Kernel: 4.6.0-1-MANJARO x86_64 (64 bit gcc: 6.1.1) Desktop: KDE Plasma 5.6.4 (Qt 5.6.0) Distro: Manjaro Linux Machine: System: Hewlett-Packard (portable) product: HP EliteBook 8730w v: F.20 Mobo: Hewlett-Packard model: 30EC v: KBC Version 91.25 Bios: Hewlett-Packard v: 68PAD Ver. F.20 date: 12/07/2011 Battery BAT0: charge: 48.1 Wh 100.0% condition: 48.1/52.0 Wh (93%) model: Hewlett-Packard Primary status: Full CPU: Dual core Intel Core2 Duo P8700 (-MCP-) cache: 3072 KB flags: (lm nx sse sse2 sse3 sse4_1 ssse3 vmx) bmips: 10113 clock speeds: max: 2534 MHz 1: 1600 MHz 2: 1600 MHz Graphics: Card: Advanced Micro Devices [AMD/ATI] RV635/M86 [Mobility Radeon HD 3670] bus-ID: 01:00.0 Display Server: X.Org 1.17.4 driver: radeon Resolution: 1280x800@59.91hz GLX Renderer: Gallium 0.4 on AMD RV635 (DRM 2.43.0, LLVM 3.8.0) GLX Version: 3.0 Mesa 11.2.2 Direct Rendering: Yes Audio: Card-1 Advanced Micro Devices [AMD/ATI] RV635 HDMI Audio [Radeon HD 3600 Series] driver: snd_hda_intel bus-ID: 01:00.1 Card-2 Intel 82801I (ICH9 Family) HD Audio Controller driver: snd_hda_intel bus-ID: 00:1b.0 Sound: Advanced Linux Sound Architecture v: k4.6.0-1-MANJARO Network: Card-1: Intel 82567LM Gigabit Network Connection driver: e1000e v: 3.2.6-k port: 80c0 bus-ID: 00:19.0 IF: enp0s25 state: down mac: 00:25:b3:7c:0d:6c Card-2: Intel Ultimate N WiFi Link 5300 driver: iwlwifi bus-ID: 03:00.0 IF: wlp3s0 state: up mac: 00:21:6a:5f:84:3a Drives: HDD Total Size: 320.1GB (5.0% used) ID-1: /dev/sda model: Hitachi_HTS72323 size: 320.1GB Partition: ID-1: / size: 116G used: 15G (14%) fs: ext4 dev: /dev/sda2 Sensors: System Temperatures: cpu: 43.0C mobo: 30.0C gpu: 65.0 Fan Speeds (in rpm): cpu: N/A Info: Processes: 174 Uptime: 1:18 Memory: 2002.2/3857.2MB Init: systemd Gcc sys: 6.1.1 Client: Shell (bash 4.3.421) inxi: 2.3.0