I had similar troubles with nrjavaserial and locks it attempts to create. I spent a day or two trying to debug it and solve problem without luck.
Instead I made a serial port provider based on a plain java library called GitHub - nyholku/purejavacomm: Pure Java implementation of JavaComm SerialPort. It has no native parts. Native access for Linux/Windows/FreeBSD is provided through JNA.
I implemented a serial provider compatible with openHAB 3.x which works for me in places where rxtx can’t. Please download it and test if you wish. You will need openhab-runtime-jna installed. This serial provider ships purejavacomm 1.0.3 aligned with JNA 5.4.0 which is used in OH 3.x.