I had the same problem... not with PCR2.1, but with a DENSO SH01.
Kess and pcmflash can connect with it without problem, hexprog not even touch it. Not factory mode or boot mode.
I'm afraid to open a SID310 from Mercedes A180, because other tools, like SMOK needs to extract password in OBD mode, then use the password in boot mode. I doubt hexprog can extract and decrypt password only in boot mode.
For the price that hexprog costs, this issues must not exist... not in this kind of ecus.
In you case, pcr2.1 is an old known ecu... almost every tool can communicate with it without problems. Personally, I use pcmflash for pcr2.1.
I hope hexprog team can solve your problem.
Regards
Pedro Dias