blank screen after logging on from locked session

[obaino]

I'm running linuxmint 19 xfce on a Dell inspiron 5570 laptop with the following specs:

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System:    Host: Dell-5570 Kernel: 4.15.0-30-generic x86_64 bits: 64 gcc: 7.3.0
           Desktop: Xfce 4.12.3 (Gtk 2.24.31) Distro: Linux Mint 19 Tara
Machine:   Device: laptop System: Dell product: Inspiron 5570 serial: N/A
           Mobo: Dell model: 0YDF7T v: A00 serial: N/A UEFI: Dell v: 1.1.6 date: 06/15/2018
CPU:       Quad core Intel Core i7-8550U (-MT-MCP-) arch: Kaby Lake rev.10 cache: 8192 KB
           flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 15936
           clock speeds: max: 4000 MHz 1: 1600 MHz 2: 1600 MHz 3: 1600 MHz 4: 1600 MHz 5: 1600 MHz
           6: 1600 MHz 7: 1600 MHz 8: 1599 MHz
Graphics:  Card-1: Intel UHD Graphics 620 bus-ID: 00:02.0
           Card-2: Advanced Micro Devices [AMD/ATI] Topaz XT [Radeon R7 M260/M265 / M340/M360 / M440/M445]

very often when I the screen is locked and I try to log back on it turns black.
My only option is to switch tty and kill lightdm to log on back again to a xfce.

Thanks in advance for any help on this issue

[obaino]

I also get the following in syslog:

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acpi INT3400:00: Unsupported event [0x86]