Is there a way to get the switch back to it's factory default? I tried using the LED display, but that doesn't work. I tried it via de purgeenv commando through rommon, but that doesn't work either.

I did a format flash and a tftpboot, but still I get a prompt to enter username and password. Or is there another way to execute some sort of password recovery procedure?

I am not sure if it works for the S3500 as it does on the controllers, but have you tried the following:

Console into the device

User: password

Password: forgetme!

(aruba) >enable

Password: enable

(aruba) #configure terminal Enter Configuration commands, one per line. End with CNTL/Z

(aruba) (config) #mgmt-user admin root Password: <new admin password>

Re-Type password: <new admin password>

(aruba) (config) #exit

(aruba) #exit

(aruba) >exit

User: admin

Password: <new admin password>

(aruba) >enable

Password: enable

(aruba) #configure terminal Enter Configuration commands, one per line. End with CNTL/Z

(aruba) (config) #enable secret

Password: <new enable password>

Re-Type password: <new enable password>

 (aruba) (config) #write memory

The above steps work as I just performed this on our 3500 via the console. Thanks for the input...

This worked for me on a 3200, up to the point that i get to change the password.  It then says that the password does not meet the password complexity requirements 

Can you print the exact message you received?

I initially thought the issue could be that a password policy had been enabled on your switch but I can't replicate an error that says "password complexity".

Here is an example of a policy when it is enabled.

!
aaa password-policy mgmt
   enable
   password-min-length 10
!

(host) #show aaa password-policy mgmt

Mgmt Password Policy
--------------------
Parameter                                                                                                 Value
---------                                                                                                 -----
Enable password policy                                                                                    Yes
Minimum password length required                                                                          10 characters
Minimum number of Upper Case characters                                                                   0 characters
Minimum number of Lower Case characters                                                                   0 characters
Minimum number of Digits                                                                                  0 digits
Minimum number of Special characters (!, @, #, $, %, ^, &, *, <, >, {, }, [, ], :, ., comma, |, +, ~, `)  0 characters
Username or Reverse of username NOT in Password                                                           No
Maximum Number of failed attempts in 3 minute window to lockout user                                      0 attempts
Time duration to lockout the user upon crossing the "lock-out" threshold                                  3 minutes
Maximum consecutive character repeats                                                                     0 characters

To recover S2500/S3500 back to factory default password (aruba/aruba123) is same as other controller; that is "password/forgetme!" works..

To use LCD to bring up factory default configuration, please do the following ..

1.> ":Maintenance" -> "Factory Default" (same as "write erase" at CLI)

2.> "Maintenance" -> "System Reboot" (same as "reload" at CLI)

Please note - same as CLI commands - "write erase" only roll back the configuration to factory default but then needs "reload" to bring up the active configuration as factory default.

I have the S3500-48P and that sucker refuses to reset to factory defaults no matter what. I even went inside and replaced the button battery which looked original, and still it wouldn't reset. This ordeal started a few weeks ago when I tried to GUI in, it would not connect and the browser kept saying:

This site can't be reached

The webpage at https://192.168.1.228:4343/ might be temporarily down or it may have moved permanently to a new web address.

The ERR_SSL_KEY_USAGE_INCOMPATIBLE browser error you're seeing is because of how modern browsers treat old self-signed certificates. Can you still SSH into the switch?

To get around ERR_SSL_KEY_USAGE_INCOMPATIBLE, try a different browser, an older browser version, or disabling RSAKeyUsageForLocalAnchorsEnabled for Chromium based browsers.