I know this is a pain. Here is a solution that I 'created' years ago that works the majority of the time.
First, let me explain what is going on:
When you join any ARMA server that uses the 'Signature Check', which almost all do, your machine will compare your mods against the server's via the 'key' file. If your HDD is fragmented, slow internet connection/issues, low RAM, or a slow PC, the check will 'time out'. Example: When it your PC checks the 'CUP' mod, ARMA is saying you have 5 seconds to do this. If your PC does in that time or less, all is OK. BUT, if it is longer than 5 seconds, it 'times out', and you get that message.
So, how does one address this?
#1 - If you have a TRADITIONAL HDD, NOT a SSD, defrag it! You should NEVER defrag a RAM HDD (SSD)! The BEST way to defrag this is to boot your PC to a USB memory stick and then defrag. Not doing this will not do a 'complete' defrag as you have files in use/open and all defrag programs will ignore them. If you have ARMA on a 2nd/3rd/etc. HDD, then you can just defrag from there. Though it still might be an issue due to files in use. Thus, a USB boot is the BEST way to do this.
One that I use is called: "Gandalf's Win10PEx64". It comes with a defrag program on it.
Google Search: HERE
#2 - After #1 is done, join a server. #1 might have fixed the issues. BUT, if not then do this silly trick:
Once you have 'boots on the ground', RIGHT AWAY, open your map and zoom in as close as you can. This 'trick' causes ARMA to run as fast as possible. There is no 'environment' being drawn, no icons on the map, and other things. Sit in the map screen zoomed in as close as you can, wait for about 3 - 5 minutes DOING NOTHING - Hands off the PC! Then resume normal play. You also can 'tell' when the 'wait' is over by watching your HDD activity light.
#3 - This is also a known issue with people having 4gb or less RAM. As an example, I had 8gb and one stick went bad, so I had to remove it, giving me 4gb. IMMEDIATELY after that, I got the same error. After I replaced the RAM, the problem went away! And I did NOTHING to my system to cause it/fix it.
>4gb = ARMA loads and processes the files in RAM - No issues 99% of the time.
<=4gb = ARMA processes the files from you HDD (thus the 'speed' issues and 'time-outs').
People and myself have been using this technique for years now and it DOES work. Does it work for everybody? No. But for at least 70% of people, it does.
Best solution? Increase RAM to > 4gb and defrag your HDD's.