@fonm After some tests I found the solution.

opera --proxy-auto-detect

Thanks a lot!

@fonm After some tests I found the solution.

opera --proxy-auto-detect

Thanks a lot!

@fonm I do not understand. I have activated the VPN at the operating system level. With Firefox I can navigate safely. In Opera do I have to manually set the proxy to be able to navigate? It seems absurd to me. In any case which proxy should I enter?

Opera version: 72.0.3815.378

This is the linux system information about my installation:

System:    Host: elisa-K54C Kernel: 5.4.0-54-generic x86_64 bits: 64 Desktop: MATE 1.22.2 Distro: Linux Mint 19.3 Tricia 
Machine:   Type: Laptop System: ASUSTeK product: K54C v: 1.0 serial: <root required> 
           Mobo: ASUSTeK model: K54C v: 1.0 serial: <root required> UEFI: American Megatrends v: K54C.207 date: 04/18/2012 
Battery:   ID-1: BAT0 charge: 42.5 Wh condition: 44.6/48.4 Wh (92%) 
CPU:       Topology: Dual Core model: Intel Core i3-2310M bits: 64 type: MT MCP L2 cache: 3072 KiB 
           Speed: 798 MHz min/max: 800/2100 MHz Core speeds (MHz): 1: 798 2: 798 3: 798 4: 798 
Graphics:  Device-1: Intel 2nd Generation Core Processor Family Integrated Graphics driver: i915 v: kernel 
           Display: x11 server: X.Org 1.20.8 driver: modesetting unloaded: fbdev,vesa resolution: 1366x768~60Hz, 1366x768~60Hz 
           OpenGL: renderer: Mesa DRI Intel HD Graphics 3000 (SNB GT2) v: 3.3 Mesa 20.0.8 
Audio:     Device-1: Intel 6 Series/C200 Series Family High Definition Audio driver: snd_hda_intel 
           Sound Server: ALSA v: k5.4.0-54-generic 
Network:   Device-1: Ralink RT5390 Wireless 802.11n 1T/1R PCIe driver: rt2800pci 
           IF: wlp2s0 state: up mac: 84:4b:f5:12:54:06 
           Device-2: Qualcomm Atheros AR8151 v2.0 Gigabit Ethernet driver: atl1c 
           IF: enp4s0 state: down mac: 30:85:a9:11:9f:4d 
           IF-ID-1: tun0 state: unknown speed: 10 Mbps duplex: full mac: N/A 
Drives:    Local Storage: total: 931.52 GiB used: 674.09 GiB (72.4%) 
           ID-1: /dev/sda vendor: Toshiba model: MQ01ABD050 size: 465.76 GiB 
           ID-2: /dev/sdb type: USB vendor: Toshiba model: External USB 3.0 size: 465.76 GiB 
Partition: ID-1: / size: 176.59 GiB used: 72.16 GiB (40.9%) fs: ext4 dev: /dev/sda7 
           ID-2: swap-1 size: 5.60 GiB used: 370.9 MiB (6.5%) fs: swap dev: /dev/sda6 
Sensors:   System Temperatures: cpu: 52.0 C mobo: N/A 
           Fan Speeds (RPM): cpu: 2100 
Info:      Processes: 213 Uptime: 1d 7h 07m Memory: 3.75 GiB used: 1.85 GiB (49.4%) Shell: bash inxi: 3.0.32 

I installed Opera with the official .DEB file.
When I activate the VPN from the system dock icon I am able to surf the internet with Firefox but I cannot with Opera.
The error is ERR_CONNECTION_REFUSED.
When the VPN is off I can surf the internet with both of them.
Firefox is configured to automatically detect proxy settings but in Opera I can't find a similar setting.
Tips?