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pay attention to steps 5, 6, 7, and 8 - this is where most people run into problems by failing to disable plugins, apps, etc.
if you are lucky - you could skip 5-8 but if you are reading this you may not have been lucky, so you might as well follow steps 5-8.
1) Ensure your vanilla 2.1 forum is working properly.
2) Upgrade your php version to php 5.3 if not already at a minimum of php 5.3
3) you can use phpinfo(_) to get info. (see the FAQ link below)
4) After you are proper version of php test your forum
5) NOW - disable plugins via the dashboard (yes via the dashboard not via config.php)
6) also disable extra applications from the dashboard (do NOT disable Vanilla or Conversations)
7) change to the default theme or bittersweet theme.
8) if you created and bootstrap.early.php, bootstrap.before.php, or bootstrap.late.php (copy them to your local pc and delete them on server. if you didn't create this files skip this step.
9) copy your .htaccess and conf/config.php to your local pc and save.
you still have not upgraded ....
try a post and signin - now that you have disabled add-ons and switched to default theme.
if no errors proceed to the following steps.
10) Download vanilla 2.2 files and follow instructions in release notes announcement and the readme.
11) verify your .htaccess is correct.
12) delete the .ini files in your cache folder.
13) run /utility/structure as instructed
14) run utility/update as instructed - if you don't get successful , rerun a few times
15) if it isn't successful - you will most likely have problems later.
16) delete the .ini files in your cache folder. (for good luck)
17) if you still don't get successful or you get a blank screen you can troubleshoot see below.
18) if you still have errors - you can try copying 2.2 files again, perhaps you didn't copy correctly.
19) if no errors, enable plugins one by one and test if no errors
20) add any applications and test for errors.
21) change your theme.and test for errors.
22) troubleshoot if necessary.
add $Configuration['Debug'] = true; to your /conf/config.php file.
if after adding $Configuration['Debug'] = true; to your conf/config.php and you see no errors.
if you get a blank screen with no errors showing you can change your index.php in your root folder.
change line in index.php
if you see an error number and an error - post the error on the forum if you don't know how to correct it.
post error message or problem.
state whether you upgraded
your operating system
your version of php
and if you followed steps 5,6,7,8
$Configuration['Debug'] = false; to your /conf/config.php file.
see FAQ http://vanillaforums.org/discussion/comment/233540/#Comment_233540 for additional help.
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.