After last upgrade Armbian on OrangePi PC Plus from version
ARMBIAN 5.73 stable Ubuntu 16.04.5 LTS 3.4.113-sun8i -  3.4.113-sun8i #16 SMP PREEMPT Thu Jan 31 08:15:34 CET 2019 armv7l armv7l armv7l GNU/Linux
to 
Ubuntu Xenial with Armbian Linux 3.4.113-sun8i - 3.4.113-sun8i #2 SMP PREEMPT Wed May 8 15:09:43 CEST 2019 armv7l armv7l armv7l GNU/Linux

the modem on port ttyS3 stopped working.

Quote

Unable to read the firmware version after six attempts

 

dmesg | grep S3
[    0.621150] uart3: ttyS3 at MMIO 0x1c28c00 (irq = 35) is a SUNXI

ls -la /dev/ttyS3
crw-rw---- 1 root dialout 251, 3 Oct 15 13:03 /dev/ttyS3

 

script.fex


[uart0]
uart_used       = 0
uart_port       = 0
uart_type       = 2
uart_tx         = port:PA04<2><1><default><default>
uart_rx         = port:PA05<2><1><default><default>

[uart1]
uart_used       = 0
uart_port       = 1
uart_type       = 4
uart_tx         = port:PG06<2><1><default><default>
uart_rx         = port:PG07<2><1><default><default>
uart_rts        = port:PG08<2><1><default><default>
uart_cts        = port:PG09<2><1><default><default>

[uart2]
uart_used       = 0
uart_port       = 2
uart_type       = 4
uart_tx         = port:PA00<2><1><default><default>
uart_rx         = port:PA01<2><1><default><default>
uart_rts        = port:PA02<2><1><default><default>
uart_cts        = port:PA03<2><1><default><default>

[uart3]
uart_used       = 1
uart_port       = 3
uart_type       = 2
uart_tx         = port:PA13<2><1><default><default>
uart_rx         = port:PA14<2><1><default><default>

 

The question is closed - the problem was in the Board OrangePi PC Plus itself.