I've just looked up the Arduino Tian. Like most of these exotic Arduino boards, they are already retired before I hear about then.
It is a dual processor board with an Atmel SAMD21 MCU (32 bit Arm Cortex M0+) processor and a Qualcomm Atheros MIPS processor. A Linux variant called Linino runs on the Atherios. It also has WiFi.
Ciao is a connectivity architecture, and in this case, the CIAO core is running on the Linux (Linino). The sketch itself, which uses CIAO services, runs on the SAMD21.
Anyway, back the the OP's problem. I guess the first thing is to try to narrow down where it is failing, in the CIAO core on the Linino Atheros or in the sketch which is running on the SAMD21.
The Linino screen shot taken just after a failure does appear to indicate that the CIAO core has run out of memory (maybe a memory leak). I'd guess now that any use of the String class in the sketch which runs on the SAMD21 would have no impact on the Linino side.
I've never experimented with these dual processor boards, Yun Tian, Uno-Wifi etc. so I can't really offer much more advice.